Im running a pretty weak pc here too, I've got a Radeon x200 and 2G of ram.
currently I have HL2, TF2, D2, Gmod10, WCIII, H:CE and Spore installed, and they all run fine.
I also had UT3 and SWG trials installed, they ran fine too.
Unfortunately, It is not powerful enough to run the best MMO ever, PS.
Edit: if you want to know what you're dealing with, just run DXDIAG.