I imagine this is your PSU. You say the PC runs fine with an X800 which doesn't require a 6pin connection, but as soon as it comes into play, you have problems. Silver power is a cheap brand and not the best.
What I suggest is that you download
Furmark or
3D Mark and run it for ~1hr with each card. If it doesn't fail on either, then it's not GPU related. In which case, download
Prime 95 and run overnight. If it can do ~8hrs with 0 errors or reboots, then we're looking at something software related. Might want to do a format + reinstall of windows to eliminate any bad drivers etc.
On a side note - love this board so freakin much! Got a Q6700 on air (Akasa 965) there at the moment running 400FSB*9 for a loverly 3.6ghz solid & <50c @ 100% load!!! - almost makes me want to downgrade to 1 of these puppies to play around with it some more!
~Bex