First thing I would try is a LiveCD linux distro, doesn't really matter
which one (I like Ubuntu for this), and see if that boots without error.

Had a couple of problems before with supposed quad core capable
motherboards that actually weren't...
Does this motherboard have the exact bios and hardware revision of the
faulty one? ie does it properly support that CPU?
Do you have the bios settings set correctly?

Is the PSU up to the job? I know you said it is, but what is the rating
of the 12v rail and is it clean...

Lee