The beep the computer makes during startup is a signal that it has detected a working video card. I don't know anything about that motherboard, but I'm thinking it's a incompalibility issue, or a dead PCI Express slot.
Eep, just read the little edit, and that's probably the issue. Most motherboards are 20 pin connector, unless they are server motherboards in which case they'll have 24 pins. Power supply could indeed be the problem.
Intel C2D E6320 / AMD Athlon X2 3800+
Gigabyte 965P DS3 / DFI nF4 Ultra-D
2GB OCZ Gold PC2-6400 / 2GB OCZ Gold PC4000
eVGA 8800GTS 320MB / eVGA 6800GS 256MB
150GB Raptor / 74GB Raptor
2x500GB / 320GB
OCZ GameXStreme 850w / OCZ StealthXStream 600w