As mentioned, the AMD 890FX chipset has 42 PCI Express x1 lanes – just to put things into perspective, the AMD 890GX chipset has 22 lanes. This allows this chip to provide a very high-end slot configuration. On Crosshair IV Formula ASUS decided to hook up these lanes like this: the first PCI Express x16 slot always work at x16, independently of how many video cards are installed; the second PCI Express x16 slot always work at x8; the third PCI Express x16 slot works at x16 when two video cards are installed or at x8 when three video cards are installed; and the fourth slot always work at x4.
So it is really important to use the first and the third (and not the second) PCI Express x16 slots when using two video cards in order to get the best performance possible.
It is important to note that motherboards from other manufacturers may use a different configuration. With 42 available lanes it is possible, for instance, to have four PCI Express x16 slots working at x8 at the same time.
The motherboard also comes with two standard PCI slots.
It is important to note that in order to fit so many slots on a motherboard without making it to be bigger than ATX form factor, there is no space between the third and fourth PCI Express x16 slots, as you can see in Figure 2. This means that if you install a dual-slot video card on the third PCI Express x16 slot you automatically “kill” the fourth PCI Express x16 slot. And you will need a case with eight slots for installing a dual-slot video card on the fourth PCI Express x16 slots (plus the video card will block access to the buttons and headers located near this slot, see Figure 2). These issues should not be a problem for most users, but if you are willing to build the ultimate gaming machine with four video cards under CrossFireX mode, you should keep them into consideration.
Near the fourth PCI Express x16 slot the motherboard has a series of buttons: overclocking (“O.C.”), which allows the motherboard to automatically overclock itself for you; core unlock, which unlocks “hidden” CPU cores available on certain Phenom CPUs; power; and reset.