I had sort of the same problem. At first, I had 512mb pc3200 ddr 400 corsair for a while, then decided to jump to 1.50gb. When I installed the ram, It didnt boot. Then it did, got to the desktop, then rebooted. Then some how or another my HDD got messed up in the process. Missing OS. So I had to re-install everything, left the 1.50gb in, and it's worked ever since. I can also over clock my system faster than before of 3.2ghz to now 3.49ghz.