Problem is, as above says, you actually need a better machine for gaming than for work.
Bare minimum today would be 3gig processor, AT LEAST 512 Ram pref 1 gig, at least a Geforce 5700 Ultra or Ati 9600XT (not SE!!).
Good news is that could be had if careful for around £500 ($I'm not sure). The main tthing to try and do (Tricky right now) is get a motherboard which will accept upgraded graphics and processor later.
For work currently use a celeron 800, 512Mb RAM 64Mb GF 5200. Its great for work and will play Counterstrike or Age Of Empires Multiplayer. Just! Actually it plays Halo multiplayer with everything turned right down.
Games machine is an XP2000+ 1Gig Ram Rad 9800 Pro. (all on the verge of upgrade city, certainly in the next year or two)
See what I mean?