Since you were an original EQ addict, I feel your plight. I'd recommend Morrowind or Neverwinter Nights, both great RPGs with amazing singleplayer action. Both can be stopped at any time.. NWN is third person (think Diablo II) while Morrowind is first/third like EQ.. amazing game that I used to play in between classes myself...
For us busy college kids, first person shooters are always great.. you can play 5 rounds and enjoy it.. with your setup you should be able to handle Counter-Strike:Source (on medium settings), or Unreal Tournament 2004.
Need for Speed Underground 2 is a great "street racing" game that your computer could surely handle, and anything from a Tom Clancy series would run well too: Splinter Cell: Pandora Tommorow or Chaos Theory...
mmm... gamess....