System Power Loss, on video playback?? help~

Hi guys, was wondering if anyone could help me out on this one!
my rig is as follows

Opteron 175
Cruicial Ballistix Tracer 2G
DFI Lanparty NF4 SLI-DR
XFX 7900GT
2*120G Seagate barracudas in RAID-0(IDE)
1*250G Seagate barracuda (sata)
Enermax Liberty 500W modular PSU

i just recently (well a few months ago) got the opteron to replace my old Athlon 3700+ and when i installed new cpu i flashed bios to the latest version available, the system ran fine, then i OCed the CPU to 2.6G and ran prime 95 3Dmark06 and memtest, no problems at all, gamed away on fear for a few hours no issues at all. then i tried playing a video, and just like that the system lost power! not a shutdown, just a complete loss of power. when i went to reboot the PC wouldnt even post till i cleared CMOS.
i have to presume that the PSU is at fault? 500W PSU SHOULD be enough though?
Id really appreciate it if any one has any ideas as i really dont want to buy another PSU
Cheers guys!

It posts fine after a CMOS clear, and sometimes if you wait 15 odd minutes itll post without a cmos clear...as for heat, its not an issue, when it was running OCed at 2.6 it was hitting about 38 C under load, zalman 9500 coolers rock the hazbar. and as for voltage it was still at 1.4V so that shouldnt have been the issue..
Im fairly sure i should just get a new PSU as it surely must be the culprit