New Mobo, Wont Post

Biostar K8M800

6600 GT

512pc2100

It powers, only if the jumper is on Clear CMOS and no display.

Double tested power supply, dont have an extra monitor.

Does it give beep codes?

Pull your memory, see if you get a no-memory error code. That verifies that the motherboard and processor are working to some degree.

May be worth your time to reseat everything.

you've got the second, 4-pin power connected to the motherboard?