I have an asus ah3450 and when i enter some games my pc restarts

i have bought an asus ah 3450 series with 512 mb of ram . of course it doesnt works on my agp 4x slot on my old motherboard so i bought a new motherboard . i have reinstalled windows i have installed all of the drivers and the drivers for that video card form it's cd . the weid thing is that under windows sp 2 if i install updated drivers from website it doesnt crashes but under sp 3 if does no matter what i do