There are some big difference (rule wise) between 2E (aka AD&D) and 3E (3rd Edition DND) and some minor ones with 3.5 (3.5 DND).
Basically: 2E is harder, where rules are reverse (armour has to be as low as possible, while your Thac0 - To hit armour class 0) has to be as low as possible as well).
3E & 3.5E have it 'normal', where both have to be as high as possible. 3.5 itself has some minor and some major changes over 3E, but in essence they're the same rulesset. Still, you probably couldn't use a 3E character in a 3.5E pen&paper game (and the other way around wouldn't work either probably). Because my game started in 3E (pnp game that is), I'm still using those rules.
Overall you need little knowledge of those rules to play the Infinity Engine games however. The only stuff that may be 'difficult' to understand at first may be spells, that's about it.
very good point. that's why i found out that nwn is so much easier to understand and to play than games based on 2nd ed.
