Need to tell a story. I built a comp with an MSI 6870 and got it posted, working fine, and playing SWTOR and other games.
I had an extra 6870 (Sapphire) and installed it with my crossfire cable. Installed drivers, etc. and everything was fine. I wasn't able to play anything on it (SWTOR, Witcher 2) because I removed my wireless card to fit the new GPU.
I was able to run 3dMark on with the new crossfire, but on the final test, the program stopped. Computer was fine otherwise.
Now I went out to buy a USB wireless adapter, and in the interim, my Bro removed one of the cards and plugged the Wireless card and try to boot up SWTOR. I should mention he was able to boot Windows, internet, etc. But when he tried loading SWTOR, the screen went black.
It has stayed black, and wont boot regardless of which GPU is plugged in (I tried each combination of cards, and all of the output slots).
I should note im running a 600w PSU which is the minimum for this set up, but like i said, before Bro went in a switched out a few things, the comp would boot fine.
BTW, if you know the Motherboard I've posted, the indicator LED's (red lights) hang for a bit on the Boot (Sata) led.
It would help if you told us exactly what you mean by "won't boot" - just how far it got. From your original message I couldn't even tell whether it powered on or not. Since you mentioned resetting the BIOS do you mean you can get to the BIOS screen or do you mean using the jumpers on the motherboard to reset the BIOS?