Was about to post this last night when my machine reset (damn this Camino chipset).
Summary (please correct me if I'm wrong):
1. You've fitted a new motherboard
2. You've used the same processor, power supply, case, graphics card and memory as from the machine that had the board blow up.
Possible causes:
1. Power supply went faulty when the other one blew up. You'd need to try it with another (known working) board to prove this. Same thing with the other components as when the undersize of the motherboard was grounded you would have had stray voltages connecting to all sorts of components (and of course the processor would have quite a current drain and hence be one of the most likely failures after the power supply).
2. Memory? - though I doubt you would have mixed SDRAM and DDRAM on the same board.
3. CMOS Battery dead - a long shot but IIRC there were some boards that were sent out with a faulty battery. Solution being change the battery over for a known good one.