Hey Ben, was just searching google on my mobo, same as you have, MSI K9N Neo-F.
Here's my setup:
AMD Athlon 64 3200+
MSI K9N Neo-F
Kingston 2GB Dual Channel Kit PC2-6400 (800MHz)
Western Digital 250GB SATA
EVGA 7600GT
SPI 400 Watt PSU
Lite-On DVD-RW
I am having same issue as you sort of, I hope... Since you were able to solve it, I think... Today nVIDIA finally release official WHQL drivers for nForce 550, I just installed them, same thing happens, completely random reboots... Also, I get memory reference errors when I close Internet Explorer quite often... I want to try the beta drivers that you mentioned to see if it helps me. Can you tell me exactly which version you used and also if it truly fixed the problem? Or if it just didn't freeze for a while and then all of a sudden started doing it again? Did you ever update to BIOS 1.3? Mine was brand new, and didn't come with D-Bracket diagnostic LED thing. I first tried OCZ cause it was cheapest, but that was huge mistake, never use them again will I... I've NEVER had trouble with Kingston, they ARE the BEST. I don't like Corsair either. KINGSTON will NEVER let you down, the one brand that is ALWAYS perfect... This is driving me nuts though, if I don't find a fix soon, I'm gonna either get the 570 Diamond, or an ASUS, something under $100.00. I normally use only ASUS but this one seemed fine at the time, I've used MSI plenty in the past with limited to no problems.
BOTTOM LINE: Please tell me what beta chipset drivers you used. I'll go to the nVIDIA site now to see if maybe there's only one beta version, maybe that will work.
Something else, before I flashed to 1.3 BIOS from 1.2, the Windows Memory Diagnostic from Microsoft passed all extended tests 100%. Now, it hangs on test #4, but memtest86 3.2 still passes 100%. I have had the random reboot issues the whole time. So I don't think the flash really did it. Hoping for a new BIOS soon, maybe that would help.
Final thing, before the random reboots, after installing XP, the first issue i get is a BSOD something like "kernel in page error". I was checking up on that issue, and it seems like a lot of people think it's related to a bad hard drive, I don't agree, while I know it's possible that my brand new HDD could be bad, I'd rather not think so, and with all the other issues this brand new socket AM2 motherboard is having, I'm sure it's the motherboard. But after that one BSOD, it does the random reboots, even when I set Windows NOT to restart upon error.
Please post your system including hard drive, thanks a bunch!
Kingston GOOD, OCZ/Corsair BAD!
AMD GOOD, Intel BAD!
nVIDIA GOOD, ATI BAD!
Everyone should thank HP for being the only ones so far to stand up to Apple's Anti-PC propaganda. Apple sucks, don't buy iPODs!
Peace.