Icewind Dale uses the AD&D ruleset, while Icewind Dale 2 and the Neverwinter Nights series use the 3rd edition ruleset. You will see marked similarities between the two games; the spell slot and memorization system is the same, there's still the same 6 attribute, and all of the same classic classes are still there. However, a lot of things have been changed around. I personally think that the 3rd edition ruleset is a huge improvement on AD&D, but it is different. All three games - IWD2, NWN1, NWN2 - are fun and have different quirks. Definitely well worth playing, especially if you picked the lot of them up in the bundle that goes on sale every so often.

The differences between AD&D and 3rd edition are innumerable. During character creation alone, you will see that the ability tables have been completely overhauled and unlike AD&D you're not gimped if you don't have a perfect 18. Feats offer you the ability to select special abilities and bonuses above and beyond what is offered by your class. Skill points let you customize what non-combat skills your character has (although many classes have severely limited options in this regard). Multi-classing also works differently in 3rd edition, and combos that used to be great are utter trash and some combos that weren't even possible before are really great.

Neverwinter Nights games are different in 2 ways

1. They use Aurora engine while Icewind dale games use Infinity engine. Even tho Aurora is a much more improved version, it also means that there are no hand-drawn backgrounds. Some people prefer the beautiful backgrounds of Baldur's gate or Icewind Dale games.

2. If i know correctly, IWD uses D&D 2.5, while NWN games use D&D3.5 . There are huge differences between two system which I believe 3,5 is much better and easier. You dont need any tables to calculate any stat.

(IWD 2 uses D&D 3.0 rules which is closer to 3,5. I honestly dont know the differences since I never had chance to play IWD2)

Also a friendly warning. NWN1 doesnt have a party system. You play a single player character and are allowed to hire one henchman who you cant directly control. You only give it commands like (attack, hold, etc). NWN2 has a full party system.

Also, You may want to check Baldur's gate out. 1 and 2 both. From my point of view they are two parts of one big game because it follows the same story and character leveling.

The differences between 3.0 and 3.5 are rather subtle. As far as the CRPG's are concerned they're all 3rd edition, and each game has its own peculiarities.

The basics are all the same, though. Same attribute tables, same classes (only Ranger was substantially changed, and it's nothing but buffs), and some changes to certain spells and feats.

NWN2 is D&D3.5

NWN1 and NWN2 have more in common with each other than with either the 3.0 or 3.5 pen and paper rules. That's the point I'm making.

Yep, and so is ToEE. I find the rules of 3.0 and 3.5 different enough that I never refer to them as D&D3.0.

Pool of Radiance: Ruins of Myth Drannor is another D&D3.0 RPG, but it sucks.

Of possible interest, since you asked about Icewind Dale and co-op play, is that there's a full recreation of Icewind Dale in NWN2 (though not its expansions), and it can be played co-op, too.

i personally prefer the 3rd edition's ruleset. it does make multi-classing more viable than the old skool D&D rules.

While that's true, it might not make such a big difference if the games are played in co-op mode. In co-op mode even NWN1 allows party play.

EDIT: Whoops, didn't notice the thread and your post were that old already, but maybe it's still relevant.

True. I should state it is for single player. If you play NWN1 on Multiplayer or a Persistant Server you can easily form parties. Which is very fun.