i rolled back my nvidia drivers and it fixed my problems. sometimes they fuck up new drivers so i roll back and it usually fixes them. i'm only using a 256mb 5600 ( on my sempron64 3100+ w 1GB ddr400) but i found after rolling back the drivers i got better results than with the newest. also if you can, up the AGP aperture size in the bios a little. that sped up my gfx a bit. dunno if it will help you though.