That is a killer video card, for that system. The CPU is very weak, like 1.2-1.4 AMD Tbird bad and it probably had PC100 or PC133 ram (not fast at all). Definite bottleneck. Tweak the settings, that system should still play AA fine so long as you aren't running too high resolution and maxing out the gfx options.
If you want good FPS AND high gfx quality get atleast a 2500+ AND or a 2.4 Ghz P4, some decent ram and overclock it. I recommend those because they are fairly cheap and won't bottleneck the system much in gaming even at stock speeds. Of course, you should overclock to get better performance, but stock with those would be okay. Its enough power to push a game and get your card working hard.
You don't need 4 ghz (or 4000+ whatever), its just cool to have and an overclockers badge of honor. While its nice, it doesn't improve gaming performance that much over say, 3 ghz when your card is getting stressed. Turning the settings up on a game will quickly make your gfx card the bottleneck so long as you have a decent CPU/mobo/ram to work with (which you don't... yet).
To sum it up, you have 3 coices:
1) Turn the settings down on the game and keep what you have (it will easily do 60 FPS in AA).
2) Get a decent, cheap system (Mobile Barton 2500+)
3) Go all out and get a killer A64 system or go for a 4 Ghz P4 system if it suits your needs better (i.e. encoding)
EDIT: You need to see what your setting for D3D and Opengl are. If you don't know, they are in the Display--->Settings---->Advanced. Make sure you have stuff set to performance or the lowest settings to eliminate that as a problem. Then tweak your game settings to the lowest they will go, except for the resolution, 1024 x 768 should be good.