I have a similar problem with a client's machine at the moment, damn thing won't POST at all, swapped out gfx cards, psu and memory. Left with processor and mobo (which I don't have spares for) - looks like they're going to need new innards for this one. FWIW a mainboard/processor kit can usually be picked up from most computer retailers at a reasonable price - I'm on the wrong side of the herring pond to advise on who you should look at, but that'd be where I'd be going now.
~S~ Jabo