I used to have an nvidia 8600 GTS. I removed it and put in a Radeon 5770 and the machine would not POST.
I put the 8600 back in an it booted up fine. I tried the 8600 in both my PCI x16 slots and it worked, I tried the 5770 in both and it didn't work. I tried using the 6pin power from the PSU as well as a 4pin molex to 6pin gpu converter, neither worked. I messed with the bios settings, reset bios settings to default, tried booting with only 1gig of ram in. No luck.