I have a Q8200 Quad Core, Gigagbyte G31 mobo, ATI 4850 1GB.
I had a OCZ 6400 800Mhz 2GB single RAM running on it for many months. I bought the same RAM and when I put it in the second slot, my windows didnt start, said 'windows is corrupt, please reinstall', then when I removed that RAM, windows started just fine.
I ran memtest for both the RAMS, no problems.
I have only 2 slots, I tried different combos with a single stick, all worked fine.
With dual channel, the BIOS shows me that 4GB is installed, but Windows fails to start. It tells me to run System Repair from the bootable CD. I ran that twice, but I end up getting the same problem.
Problem solved. Turned out RAM was faulty. The thing, I didnt run the entire memtest, just ran it till 60% and got no errors. When I did a full test, is gave me errors when it reached 91%. I immediately went and replaced it, and now it works like a charm.!!