In my opinion nForce2 motherboards are cheap and nothing but a pain in the --I'm attempting to get by the auto-censor and should be beaten after I re-read the rules-- ... the majority of users usually have to buy 2 - 3 boards before getting one that actually works (doesn't matter if its Abit, Asus, or Epox ... there all same)
I betting though that you missed something in the BIOS related to the RAM timing (or your being to aggressive with it) and thats whats causing it to crash ... I'm almost certain of it. If its not that then its the motherboard. (Take that one stick of DDR to a friends house and swap that stick of RAM into his machine and take your pick of memory-checking programs and run that and see what results you get.)
btw ... you do have it inserting into the correct DIMM slot right? Since your not running Dual-Channel setup ... it does recommend a certain slot to use (usually).
It can't be heat related ... most of the motherboards today have a self shutdown if it senses the CPU getting to hot. I imagine you have that set or at least I hope you do. The problem of crashing would get progressively worse too ... so you would be able to get farther into the OS if that were the case ... it would just get to the point of freezing at the POST screen.
Why don't tell us the following :
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CPU (give us an idea of what the temps should be)
Heatsink (link to it if you can)
Motherboard (link to it if you can)
RAM (link to it if you can)
If not worried about the PSU

also the actual temps, voltages, and your RAM settings. We need details or its nothing a but guessing game.