I keep coming back to Diablo 2 after three(?) years. That's probably the highest longevity of any game I've played. Not necessarily a true RPG, it still fills that niche for me as a nice counterpoint to our weekly tabletop sessions.
Daggerfall was much fun to just play. I never finished it, but it held a lot of my attension. Having just picked up Morrowind, I expect it to be at least equal, if not superior, to it's predecessor from what I've seen. Battlespire is in the same family of games, and has multiplayer support, but it's more action/Diablo-esque than the other two.
I wasn't really drawn into Baldur's Gate. It honestly bored me more than a little. I keep hearing good things about it and reinstalling it, but it just doesn't float my boat. For that reason, I've stayed away from related games.
I finished Dungeon Seige and generally enjoyed it. I am glad, though, that I borrowed it from a friend, rather than dropping my own $50. I have no real interest in replaying it.
Despite its flaws, I found Temple of Elemental Evil to be quite a bit of fun. You'll have to be willing to deal with some bugs (some are major), but they are somewhat reduced by installing the patch -- especially the latest one, by my understanding.
For reference, I've got a AMD Athlon 1Ghz machine with an above average, but not "serious", video card. If you can match that with your old machine, any of the above will run peachy (okay ToEE will run slowly, but that isn't really the fault of the computer). Daggerfall and Battlespire do seem to have compatibility issues with Win XP, though.
My top picks would probably be Diablo 2 or Morrowind, in that order. If your computer is a P2 or lower equivalent, I'd probably recommend Daggerfall or Battlespire.
