Baldur's Gate II: Shadows of Amn is definately a must-own for any fan of RPGs. It features over 100 hours of gameplay, an excellent storyline, memorable NPCs, wonderful character interactions, dozens of subquests, and addictive gameplay. You should also pick up the expansion pack, Throne of Bhaal, which is almost large enough to be a seperate game.
Icewind Dale II is another fun game. It does a good job of adapting the 3rd Edition D&D rules to a game engine that was obviously designed for the 2nd Edition rules. Gameplay is very similar to the Baldur's Gate games, but Icewind Dale 2 is a lot more combat-heavy and less story-heavy than the BG series.
Neverwinter Nights is also great. It lacks the depth of Baldur's Gate, but does a good job of converting the D&D 3e rules to a real-time system. In addition, there are literally thousands of user-made modules out there (many of which are conversions of classic AD&D modules). If you have friends who play Neverwinter Nights, the multiplayer mode can also be incredibly addictive.
Later this year, keep an eye out for Greyhawk: The Temple of Elemental Evil. It will be the first D&D computer game to use the 3.5e rules, and claims to be the closest conversion of the D&D rules to video game format ever; combat will be turn-based, and even the item creation feats will be making it into the game! It looks to be very cool.