Even I have similar problem. I had two 550 Mhz based 512MB DDR2 RAM's, I replaced it with two 667Mhz based 1 GB DDR2 RAM's. After that I am regularly getting restarts (sometimes after logging on, sometimes during the initial boot itself).
Are 1 GB cards not supported in the board or is there some configuration needed?
Once I replace them with the original RAM's this issue is resolved. If I use one 1 GB card and one 512 MB card, it restarts sometimes and sometimes not.
I am quite frustrated as I cannot utilize my new 2 * 1 GB RAM cards.
I have had to reinstall Windows fully to resolve this issue as my file systems files got corrupt initially because of these restarts. Can someone please help?
My problem is resolved. I did a memory test of my RAM by downloading the Memtest tool from below location (Burn the ISO image for creating bootable CD (Windows - zip) in a CD to run MemTest).
One of the RAM's happened to have errors, that was the problem. When it was replaced with a good one, this issue was resolved and I am running the 2 GB DDR2-667 based RAM's fine on my board.