Since you've tried so many different things, I would take the entire PC appart and add parts one at time until the problem occurs.
Presto: there's the part that is at fault. If the barebones mobo/cpu/memory combo cant get the pc to boot, you'll have to try a different one of each. Is there any beeping? Does the PC even post?
Everyone:
Check my recommendations here and correct if wrong. I think a mobo/cpu/mem is all thats required to boot a pc right?