I'm pasting a paragraph from the review regarding this issue:
Obviously the major differentiating feature of the P5W64 WS motherboard versus other motherboards based on Intel chipsets currently, is its additional 24 lane, 3-port PCI Express switch that Asus used to bring out additional PCIe connectivity for the chipset. The IDT 89HA0324PS PCI Express switch utilizes a single X8 connection to the 975X Northbridge and then in turn fans-out a pair X8 PCIe connections downstream to the expansion slots. So, in total, this chip takes the already existing 22 lanes within the 975X to a full 30 available for graphics and PCIe expansion.
The four full-length slots on this motherboard can be configured in either an X8,X8,X4,X8 configuration or X16,X0,X4,X0 (slots 1-4 consecutively). So in short, in addition to a standard dual graphics ATI CrossFire setup in the first two slots, you also get an additional X8 enabled full-length slot available for higher function cards like video cards, RAID controllers etc. On a side note, Asus retains two of the available PCI Express lanes for the dual PCI Express Gigabit Ethernet controller that is on this board, thus only the remaining 28 lanes are configured in expansion slots.
As they say, this board allow 30-2=28 lanes for the PCI-E. So x8,x8,x4,x8 is understandable since it equals 28 lanes. However, 16x,0x,4x,0x equals 20 - so where are the other 8 lanes?
Further more, in this review it is said that the 975X chipset usually allows only up to 22 lanes. That would mean that no 975X chipset board is actually able to deal with FULL double 16x SLI(in that case 32 lanes would be necessary).
If this is so, how come I never heard of this being a very significant disadvantage of the 975x chipset?