agp: sisoft sandra tells you your agp questions. (guru3d.com)
try:
-installing updated drivers from inno3d's website.
-patching the game(s) in question.
-updating to dx9.0c and WinXP sp2
-put your card back at stock speeds (memory is probably too high. should be: 2d: 270mhz/550mhz 3d: 325mhz/550mhz)
-try the omegadrivers for your card (omegadrivers.net)
-Use drivercleaner3 (guru3d.com) for every update/install of different drivers.
-install updated chipset drivers for your motherboard.
-repoting back with more specific details about this case(software using/used/OS/game) and your detailed system specs.