I bought an XFX Nforce 780i Motherboard about 6 months ago, and I ordered 4 sticks of OCZ DDR2 PC2 6400, Part Number: OCZ2N800SR4GK (2gb each=8gb) with it. The first number of weeks that I was using the MB and RAM, I was having constant problems with the computer not posting to Windows and getting a lot of BSOD. Being that I'm not very computer savvy, I tried to trouble shoot what the problem could be for a number of weeks, but I still couldn't figure out what was wrong. I decided to try and remove 2 of the 2gb sticks, and ever since then (running at 2x2gb=4gb) I haven't really had any problems with my computer (it posts to windows fine now and I've only had a couple BSOD since then ) I assume I'm just running the system with the wrong speed of RAM, but again I don't know much about computers, and what RAM is compatible with my MB. I've tried going into Bios and setting the Voltage up to 2.1 (as is recommended by the RAM) but that still doesn't seem to help. I was thinking of just buying some different RAM for the MB, so I can take advantage of the 8gb it can support, but I'm not sure what make/speed I should be looking at.
I've seen alot of motherboards refuse all 4 DIMM sockets being populated. I sold one of these motherboards a couple of years ago and it's been working well. Have you checked the timings to make sure they match what is specified on the RAM?