Computer won't POST after installing GPUs (5870 and 7970)

[RenegadeMind]

I have 2 computers and 2 GPUs. On both computers they will not POST after hooking up a GPU (5870 and 7970).

Has anyone ever seen this before? I haven't, and I'm at a loss as to what to do.

Everything is hooked up fine. Power, well seated GPU, etc.

All other components are in perfect working order.

The GPU fans go on, but computer won't POST and won't boot.

Any ideas?

[mdmx]

Do the GPU fans hit 100% and just spin with nothing on the screen?