I ran a very similar system, a Emachines T2200SE with some upgrades to a AMD Athlon XP 2400+ like yours, with a full gig 512 X 2 of PC2100, with a Radeon 9800 pro 128 MB video card. That was a decent set up, ran UT2004 fairly well, on high settings 1024X768 I constantly go about 45 FPS, Half Life 2 ran ok, high settings, constantly over 30 FPS, no AA or AF on that game though, just std triliniar filters, but it still looked nice. Doom 3 and Far Cry could only run at medium quality settings at about 800X600 if you wanted to run about 30 FPS. By the way, a full gig of ram realy seemed to aid to shorten the load times in all of the newer games.
Its a decent little gaming set up, not hard core, but serviceable. You may have to set up some of the newer games to run in medium vs. high settings, and maybe at a max of 800X600, but at the end of the day, that still looks better than any current gen console game does.