Hurdles with NVIDIA and 3-Way SLI on Ivy Bridge
Users who have been keeping up to date with motherboard options on Z77 will understand that there are several ways in order to put three PCIe slots onto a motherboard. The majority of sub-$250 motherboards will use three PCIe slots in an PCIe 3.0 x8/x8 + PCIe 2.0 x4 arrangement (meaning x8/x8 from the CPU and x4 from the chipset), allowing either two-way SLI or three-way Crossfire. Some motherboards will use a different Ivy Bridge lane allocation option such that we have a PCIe 3.0 x8/x4/x4 layout, giving three-way Crossfire but only two-way SLI. In fact in this arrangement, fitting the final x4 with a sound/raid card disables two-way SLI entirely.
This is due to a not widely publicized requirement of SLI – it needs at least an x8 lane allocation in order to work (either PCIe 2.0 or 3.0). Anything less than this on any GPU and you will be denied in the software. So putting in that third card will cause the second lane to drop to x4, disabling two-way SLI. There are motherboards that have a switch to change to x8/x8 + x4 in this scenario, but we are still capped at two-way SLI.
The only way to go onto 3-way or 4-way SLI is via a PLX 8747 enabled motherboard, which greatly enhances the cost of a motherboard build. This should be kept in mind when dealing with the final results.