Ok, you have to eliminate some factors here. Only have the motherboard, ram, processor/heatsink, video card, and power supply. Don't have anything else hooked up (hard drive, cdrom, floppy, etc). None of the other stuff is required to post. Have the motherboard sitting on a table with the processor installed w/heatsink, memory installed, video card installed and the power supply hooked up to the motherboard.
To attempt to start the system, just use a metal flat tipped screw driver and touch the two pins that the power button normally connects to. It just takes a split second of contact and the screwdriver does not need to be held on the pins for the system to remain running, ie: touch the pins together and it should boot up. If this does not work, chances are that you could have a bad chip, ram, or motherboard. I am betting that you are shorting something out though while you had the board installed in the case because 2 dead motherboards just doesn't happen too often.
I'm quoting myself from another thread. Try this and see what happens.