Asus P5B - Manual (Page 50)

2-22 Chapter 2: Hardware information
• We recommend that you install a graphics card on the primary (blue)
PCI Express slot and install other PCI Express devices on the black PCI
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.