I've recently experienced the same problem on my P5N-E SLI board. I've got an overclocked E4400 with 2GB RAM (2 x Kingston KVR800D2N5/1G) I wanted more ram in the system so I thought I'd just buy another 2 to fill the slots. This simple action was the result of many frustrating hours on the web.
Finally the problem was resolved by setting the voltage to 2.3v (for the ram) the rest of the voltages at at auto, the rest of the settings on auto, the speed the ram runs at is 800mhz. I've tested this with memtest86 for 3hours straight without a problem (as supposed to when the voltage was at auto or 2.0v, the errors numered in the 100000+ bracket)
I hope this solution helps someone with the same problem.