Boot problems

I recently put together a system and am having some problems. Here are the specs:

Motherboard: ECS 671T-M
CPU: Intel Q6600
RAM: PNY 2GB PC5300
Hard drive: Seagate 300GB SATA
Video Card: GeForce 6750 GX2
DVD: LG 30x RW
Power Supply: 500W Antec Basiq

The problem I'm having is that when I first put the system together, it would not reliably post. In fact, it would post less than half the time. Usually, the keyboard lights would flash after about 10 seconds and the DVD drive would blink, but it wouldn't beep and the monitor would get no signal. Also, even when it did post, only the onboard video would work, I never got the GeForce card to send anything to the monitor. Then I tried removing the video card and everything worked fine.

So obviously the video card is the problem, but I was wondering if anyone had thoughts as to what exactly the issue might be. The card has been tested to work fine on another machine so I don't think it's a bad card. I was thinking it could be that it's not getting enough power, but the GeForce documentation recommends 500W, which is what I have. And the motherboard documentation says it supports 16x PCI-Express, which is what the video card is.

If anyone has any idea as to what might be going on, I would appreciate it. Is there some other reason that the video card might not be compatible with the motherboard? Thanks.