Anisotropic filtering and AA are more CPU requiring than graphic card issues, so here are a few pointers and questions:
Does it happen with only one game or all of them?
What resolution were you playing the game at: try lowering it as this will also free some CPU usage.
Make sure you have the latest drivers installed for your video card (it might help a bit).
Was the game completely installed or minimum (always go for complete. CD ROM access a lot slower than HDD access)
What was your AA sample setup at 2X, 4X etc.. (try lowering it or better, in the video card control panel, select application preference wich is usually 2X but is optimized for best CPU/video card cross performance)
To increase your framerate, make sure that your Vsync (vertical sync) in the video control panel is off as well as in the game option menu.
Defrag your hard disk
stop all resident programs running such as MSN etc...(right click in the system tray icon on your programs icons and select shutdown or disable etc...(Do this also with your antivirus protection and firewall, just don't forget to turn them back on when you get back on the net) You can also prevent them from loading during boot by doing the following:
Under start, select RUN then type MSCONFIG.EXE, select setup tab and uncheck the programs you want to prevent from loading during boot
Is your AGP aperture size setup properly? with 512 M of RAM, it should be set to 256 (you can change the apperture size in the BIOS by hiiting delete during boot)
You may also want to increase your virtual memory as it might help (DO NOT change the minimum value as this might prevent your computer from loading properly, just increase the MAX value)
Last but not least, try running your game from a fresh boot. This will ensure that all your RAM is free
Hope this helps.
P.S. What game were you playing?
