Fallout 3 Skills Guide
Barter: Charisma-based skill
Raised By: Tales of a Junktown Jerky Vendor
Barter raises the price you get for sold items in Fallout 3. One of the most useless skills in the game. I never put points into the Barter skill, as the benefits are slow to build up. You'd have to put an entire level's worth of points into this skill to notice much of a difference on your return price when selling. Don't bother with this skill unless you are nearing the level cap (20) and don't have anything else to put it in. Every other skill gives you a better return for your investment. Also, at the end of the game you'll be swimming in caps and Barter will become completely useless.
Big Guns: Endurance-based skill
Raised By: U.S. Army: 30 Handy Flamethrower Recipes
Putting points into the Big Guns skill, like other weapons skills in Fallout 3, will increase your accuracy and damage. The accuracy bonus is not limited to VATS. If you use the minigun with both low and high big guns this dynamic will become more apparent. With higher big guns, the shots will form a tighter cone, granting better damage at increased range. Big guns deal massive damage in Fallout 3, and can even work well for those who haven't invested many points into the skill. The minigun is a good example of this, as at close range the gun is accurate for characters with both high and low proficiency in the skill. Of course, if you want to get the most out of guns like the flamer and minigun, you'll need to invest points into this skill as the damage increase will be vital later.
Energy Weapons: Perception-based skill
Raised By: Nikola Tesla and You
Energy weapons were my favorites when I wrote this guide. Higher skill in this weapon type raises accuracy and damage when using them. I recommend you keep this skill under 60 if you want to take the Cyborg perk, 70 if not. It will raise energy weapons by 10. Tesla armor will also give you another 10 points in this skill, and of course you can find enough skill books to get you the rest of the way as you progress through the game. The energy weapons bobblehead will also give you 10 points in this skill, but only if you progress through the story to the next-to-last chapter. So, there are 30 points that you can get for "free" (cyborg is a powerful perk), without even taking into consideration the skill books you'll find. This means you can put points into other skills.
Explosives: Perception-based skill
Raised By: Duck and Cover!
Raising this skill will increase the potential damage of thrown grenades and placed mines. You'll also receive a longer countdown before a mine detonates, giving you a chance to get away or disarm it. Mines can be disarmed without any skill in explosives, but certain traps will require proficiency in this skill to be disabled. Grenades are very powerful in Fallout 3, with varieties suited to both living and robotic opponents. You'll need a minimum of 25 to use the atomic bomb in Megaton (for The Power of the Atom). Therefore, no matter what you think of the skill you'll need 25 in order to get a house because your decision on the bomb leads to either residence in Megaton or Tenpenny Tower.
Lockpick: Perception-based skill
Raised By: Tumblers Today
The lockpick skill determines what level of lock you can attempt to pick in Fallout 3. It's got to be one of my favorite skills in Fallout 3. The locks range in difficulty: Very Easy (0), Easy (25), Normal (50), Hard (75), Very Hard (100). You'll need a bobby pin in order to try to open the lock. Watch for merchants carrying these, because running out is always a bad thing. The higher skills don't increase your chance to pick the lock, only what level you can try. In order to open the lock, you'll need to pass the lockpicking mini-game, which I have not yet mastered. You'll eventually get a feel how far you need to turn the lock in relation to the bobby pin. Having high points in this skill is recommended for everyone, because even low level areas occiasionally feature high level locked doors that protect many treasures.
Medicine: Intelligence-based skill
Raised By: D.C. Journal of Internal Medicine
Medicine skill increases the amount of health that is healed when using a Stimpak, as well as the effects of Rad-X and RadAway. There are also several points in the story in which you can use Medicine as a dialog choice. I recomend everyone get to at least 40 because of this reason. Overall, this skill will save you money in the early game by making stimpaks more effective.
Melee Weapons: Strength-based Skill
Raised By: Grognak the Barbarian
Taking points in melee weapons raises your damage with them, pure and simple. If you go this route, be sure your character has a high strength because it will help to increase the damage further. Melee characters can be powerful, but it's certainly not something I'd recommend on the first play of the game because there are times when you'll be shot at by multiple enemies. For this reason many are well off taking some points in some form of ranged weapon to help them out of sticky situations.
Repair: Intelligence-based skill
Raised by: Dean's Electronics
Your ability to repair items is capped by the repair skill. With a higher repair skill, you'll be able to get more from each item you sacrifice to repair another. The cap increases with level, eventually allowing you to repair an item at full strength. Fully repaired items deal more damage, and this is often far beyond what the damaged items you find in the waste can do. Every character should have points in this skill, because repairing armor can get expensive. You'll also be able to repair items far better than any vendor in the game, and this will cost you only the damaged item you use in the repair. Repair is used very little otherwise in Fallout 3, but the benefits of investing in this skill will be evident the first time your items get damaged.
Science: Intelligence-based skill
Raised by: Big Book of Science
Another of my favorite Fallout 3 Skills. Like lockpicking, Science skill will raise the level of terminal that you can hack. It won't help you in hacking the terminals, only unlock your ability to attempt them. The science mini-game can be tough, but I find it to be fun. You should usually save your game before you attempt a terminal, unless you're very confident as you could get locked out permenantly. This can even affect some quests if you fail. The science skill will also come into play in multiple quests, so higher skill will help you with extra dialog choices. Overall, this is a must for every character as the loot you can gain from hacking terminals will make you more powerful.
Small Guns: Agility-based skill
Raised By: Guns and Bullets
Higher points in small guns raise accuracy and damage when using them. The accuracy bonuses are more evident when using a weapon such as the sniper rifle in non-VATS mode. The gun will sway less as you aim, allowing you to make those nice sneak criticals at a longer range. The damage boost is nice, allowing you to kill enemies faster. I like some skill in small guns for all my characters because the variety of weapons allows for much tactical consideration. Need up-close stopping power? use a shotgun. Long range sniping? Whip out your reservist's rifle. There are more small gun varieties than any other weapon type, and in general the guns themselves are more plentiful. You can't go wrong with some skill in Small Guns.
Sneak: Agility-based skill
Raised By: Chinese Army: Special Ops Training Manual
Higher Sneak reduces your chances of being detect while sneaking. Sneaking is useful to everyone. Melee characters can get closer to the enemy before they engage them, and even score critical hits if they can sneak in from behind. For ranged weapons, sneak will allow you to make critical hits as long as you can sneak close enough to gain an acceptable hit rate. You'll only really be able to knock out one enemy with sneak, but that's one less enemy to worry about! If you're ranged, pick the nastiest enemy you can and aim for the head. This works like a charm.
Speech: Charisma-based skill
Raised By: Lying, Congressional Style
Speech is used in dialogue choices in Fallout 3. There are many applications. Most quests have some form of speech check you can pass to either sweeten the deal or even skip certain quest objectives entirely. Despite that it is useful for everyone, points in speech are not crucial to me. You could cheat and re-load your game to try speech attempts over, or even not care about many of them. You can pass speech checks to get better deals for certain quests, like the Nuka cola challenge, or even the quest to turn in Sugar Bombs to the ghoul in Seneca station.
Unarmed: Endurance-based skill
Raised by: Pugilism Illustrated
Unarmed has changed a lot in Fallout 3. In the old game, you had certain special moves you could use that took more AP and dealt more damage, had higher critical chances etc. In Fallout 3, you'll mostly do the same attacks but can target body parts by aiming at them. Unarmed doesn't mean necessarily 'unarmed' but can include weapons like brass knuckles and the mighty deathclaw gauntlet. Points in unarmed will raise the damage you do, and endurance increases this stat, but Strength will not affect the damage you do as it does with melee weapons. You should really consider having some form of ranged skill even if you know you will use this a lot.
I myself never got around to playing the DLC nor New Vegas but hope to one day.