Are you trying to run an ATI HD 4870 x2 with an ATI HD 3870?
If you are, it will not work.
Here is a chart that details what cards will work together in crossfire:
If you are trying to run the 4870 x2 by itself, try putting it in the other PCI-E slot - I've read some people have had success when trying that.
Other than that, you could always to flash the motherboard BIOS, as I have read that some video cards will not post without flashing this board with old BIOS
Hello guys , i m using only the 4870 X2 , my its the lasted bios , and in other PCI has the same issue , i m using Windows 7 64bits .
If you are tring to use two cards in crossfire you have to use a bridge. Or no crossfire