Well for one, yes a dual core will draw more power than a single. The wattage you are drawing with the new setup is close to maxing out a 450W PSU, but it "should" be able to take it.... Unless...

Larry, here is my suspicion. You have a loose connection to one of your hardware devices, which in turn has a corrupt/conflicting driver. My assumption is that the corrupt driver keeps the computer...

Derp, considering your situation, I would start pulling out parts from your "busted" machine and testing them in another WORKING machine... To see if everything works individually.

~Mike

Derp, does your mobo have built in graphics?

I would take out all of the cards too.. soundblaster, all that extra stuff... Try to get it to boot with only PSU, mobo, and HD. That narrows it down...

Derp, if I were you I would strip the system down to bare minimals like I described earlier. I usually do this anyways when I change out a motherboard. Every time I have just thrown a machine...

Definitely sounds like a loose connection. Make sure your ribbon cables are tight into the mobo and the HD's.

~Mike

I would try taking out your memory sticks and re-seating them... (just a shot in the dark)

Then I would unplug the power from EVERY piece of hardware that isn't needed to boot up... CD/DVD...

Do you hear any 'beeps' when its failing to boot?

Cheers :)