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