Specifications

ASUS P5B Premium 2-21
We recommend that you install a graphics card on the primary (blue)
PCI Express slot and install other PCI Express devices on the black PCIPCI
Express x16 slot..
In CrossFire™ mode, install the ATI CrossFire™ Edition (Master) graphics
card on the primary (blue) PCI Express slot.
Install two ATI graphics cards from the same GPU family that support
CrossFire™ mode.