No kit, just an allround rogue.

It's been forever since I played BG2 though, and I have no clue what the stats to aim for are. I know Dex is key, but do I need STR for extra damage (like in 3.5) to add on backstab? What INT and WIS is good to strive for, for the rogue skills? And what stats influence which saves?

I'm sort of lost :p Help?

at least 18 dex and 16 con are most important. Maxed out strength can also be useful if you don't want to rely on strength girdles. For thieving skills int, wis and cha are useless, wis is the least useful stat, only lore is affected. I'd take decent int however so you can use scrolls once you gained the UAI HLA. Maxed out cha is nice for dialogue options, but you can have someone else lead your party as well.

Optimal build: Halfling (because of the big saving throw bonusses) thief with stats 17-19-16-11-10-18, but even stats like 10-18-16-9-3-3 are sufficient.

Halflings also get 19 dex and roleplay well as thieves (imho). BTW, the slower thieving skill progression of Assasains and Bounty Hunters is pretty much negated by the relatively high level that you begin the game at, as such there isn't really too much of a disadvantage of taking one of these kits.
As kmonster said, Dex, Str and Con (in that order) are the stats to max. I cant exactly remember if the strength bonus get multiplied to a backstab, or just added at the end, but it's useful either way just because of the to hit bonus...and because the other stats don't do too much.

I would recommend the Assassin kit, both for the neat role-playing aspect, and also ofc their extra backstab multiplier. But then again, I'm a huge backstab fan, especially in BG2.

Allround thief is a very good choice though, very versatile.

Halflings make very good thieves, though any race will do fine. I personally would go with Elf or Half-Elf since I simply don't like Halflings :)

I know you said plain thief, but I would recommend you look into the possibility of a dual-classed Fighter/Thief (Kensai or Berserker preferably, but that's kind of powergaming) or multi-classed Fighter/Thief.

Thief is one of my favourite classes in the game though, either with a kit, dual/multi or just plain Thief.

The stats listed above are very good. I might drop some strenght and put on a girdle/gauntlets and put some more points in Intelligence.

spetznaz is right, adding a second class in there (multi or dual) is the way to go. You won't be in chapter 3 and your character will basically be the same as he will be until he gets HLAs. At least adding some fighter, cleric, or mage to this class spices it up a bit.

I played a Monty Haul BG2 game once with a Wizard Slayer/Thief. Once you get UAI it is fantastic! Take THAT, Irenicus!

I'm of a slightly different opinion regarding stats, if you want a powerful build. I would tend to avoid halflings, since their strength can't hit 18. The rules are not like 3.5, where every little bit helps. Only the more extreme scores get bonuses or penalties, and they change quickly (every point rather than every 2 points). So, you should probably tend for an all or nothing approach. If you take 18 in strength, you'll have an opportunity later to take it up to 19 and get great bonuses. Otherwise, you'll want a girdle of giant strength, at which point your original strength becomes meaningless.

Wisdom is actually more useful than intelligence, since it affects saving throws. Intelligence does nothing for you if your not a mage. You get the same lore bonus for wisdom that you get for intelligence. Charisma is good for getting low prices in stores, but there are plenty of magical items to boost it.

So, high dexterity, 16 constitution, as much strength as you can afford, and the rest in wisdom. Physically, the strongest race is half-orc, which lets you start the game at 19 strength -- which is way, way stronger than 18.

For a power-gaming build, you might consider a half-orc fighter-thief. Thieves top out rather quickly in BG2. Multi-classing gives you more headroom, better saving throws, more hit points, better THACO, more HLA opportunities, more attacks per round (a major plus), more learnable weapons, more weapon proficiencies, ability to specialize in weapons (allowing some nice bonuses, including an extra 1/2 attack per round beyond the bonus for being a fighter), and a few other nice perks. The only drawback is that your thieving skills will advance a bit more slowly -- no big deal given that you'll have plenty of available magic items to boost thieving skills (boots, cloaks, rings, potions, armor).

Bombur: Wisdom doesn't actually affect Saving Throws. It's supposed to, but it doesn't.

Intelligence affects how long you're held in a maze... though that might not be very important. Unfortunately the way cRPG's work, yea a rogue should go for dex/str/con and specifically in BG2, str is more important, since dex only effects thieving skills and thieving skills don't need to be enhanced to hit 100%+ at the start of the game...

Of course if you want to roleplay it, probably going with 10 str, high dex, decent con, high cha would be the way to go. high int too, low wis, as a rogue should be smart enough to get into trouble, and just unwise enough to avoid it.

I forgot about the maze thing. It's been so long since I've been trapped in one. Nothing ever lives long enough to cast one on me. And if the game really doesn't implement the wisdom bonus to saving throws, then there's no point in taking that.

From the look of the question, I figured Faragon was going for power (damage, skills, saves, etc.). For role-playing, intelligence is the traditional (i.e., 1st edition) secondary stat for thieves.

If your protagonist gets mazed it's game over (same as player death or imprisonment), hence wisdom is not useful for the protagonist in this respect. Also, very few enemies actually cast this spell, and when they do it is unlikely they'll manage to hit your thief protagonist (especially if he's backstabbing).

My 19 dex suggestion was mainly a role playing one, but it will net you an extra +1 to hit (over 18 dex), and +5% to all thieving abilities (pick pockets, open locks, find traps, move silently, hide in shadows and set traps). Though yeah, given the fact that you'll max all your thieving skills pretty early, the +3 to hit, +7 damage and +2(+5) hp from 19 str and con for a half orc is probably a much better option from a powergaming perspective....a half-orc thief though o.O - not really my style.

I decided to go with a thief/wizard for the sake of versatility. (fighter doesn't add much versatility, while wizard adds those chunky invisibility spells and all that :p )