I've got the following computer and I've just put in a new CPU QX9650 and I've run into this problem since making this hardware change.
Whenever I put more than 1 of my 4 sticks of ram into my machine it wont load an OS. It'll go through the BIOS but BSOD on windows load. It also wont let me install an OS from disk or boot into Linux.
I've ran memtest with all 4 sticks in and I get 10k+ errors on test5. Each stick of ram on it's own is fine and functions properly. I only have problems when all 4 sticks are in the machine at the same time.
Mobo: Asus P5B 2104 BIOS
RAM: 2xPC25400 DDR2 , 2xPC2 6400 both OCZ.
With mixed speed ram sticks, check the speed that the motherboard is trying to use. Maybe you have to set those manually to the lowest common denominator.
Is your BIOS up to date? I had a Gigabyte motherboard and found that I was several versions out of date, and one of the newer versions had a change related to how it works with ram.
I have read that it is harder to overclock with 4 sticks than with 2 sticks. For the machine that I was working with I had to lower the overclock a little bit. I did determine that even though I bought 4 sticks of ram as a set, one of them was a problem and the machine wouldn't boot with it in place. I used some other ram and got it to work.
2 sticks of the same speed will boot it appears but how do I check what speed the BIOS is picking up my RAM to be? I've never really overclocked so I'm not sure how to check. I've googled around about my motherboard but couldn't find anything about detecting what the RAM speeds are! Thanks.
Okay so I figured out how and set DRAM to 667 and now it freezes up when entering windows. Also it's telling me the CPU is unknown, update BIOS to unleash full power but my BIOS is the latest version AND my motherboard does support the CPU!