Oh, and I just checked and a computer will power up with no RAM at all and an unpowered gfx card. I'm not sure it would power on without a CPU, and am too lazy to check.|
Basically, what you need connect to the motherboard to be able to turn it on are the long power plug plus the separate 4-pin one, the power switch plus the CPU fan or power led to verify it's working. I'd say the CPU has to be installed as well. With this, you should have a computer that will "turn on", start spinning the PSU/CPU fans and light up the power led.