Usually if its a Dual x16 slot, you'll get 1 slot at full bandwidth and the other slot is running at x8 or it will run 1 x16 slot and then the other, you will have to set up a jumper and then it will run 2 slots at x8.
There are two PCI-E 2.0 x16 slots, both offering full x16 bandwidth for dual graphics cards. The remaining slots are three PCI-E x1 (the black slot is reserved for the supplied SupremeFX II ADI based 8-CH Audio card), and two PCI 2.3 slots, placed in between the PCI Express slots. The wide PCI Express slot spacing should help airflow, especially with high heat producing cards such as the AMD HD 2900 series. Underneath the lower PCI slot, we have a fan and thermal probe header. Further to the right we have an on-board 1394a port connector and power and reset switches. One of the Internal USB connectors is placed over the lower PCI Express x16 slot; the other two internal USB connectors are situated to the right of the power and reset switches.