|I won't even ask why you're running DDR2-533 with an AMD system or why you're running two crappy cards in an SLi config.|
Remove all the RAM but one 512MB stick, remove the SLi config & just use one card. Manually configure all the BIOS settings, then test that one stick of RAM with memtest86...one complete test series should be good enough. If any errors are found, install a different stick & run memtest86 again. If no errors are found, try installing XP.