How It All Works
The changes in architecture for Sandy Bridge made it so that PCI Express lanes are all handled by the processor. The PCI-E lanes coming out of the CPU get split, running eight lanes to the first PCI-E slot and then another eight lanes into the switch chip. The switch then looks at your configuration to control bandwidth to a single x16 slot (routing the lanes back to pair up with the eight coming from the CPU) or as dual x8 slots (routing the lanes to the second slot).
Naturally, this means that newer Gen 3 speeds also would have to come from these lanes, but unfortunately, support for the Gen 3 won’t officially happen until Intel’s Ivy Bridge processors hit the market in early 2012. The circuitries inside the current Intel 6-series chipset motherboards are theoretically capable of a PCI-E 3 connection, but as the Gen 3 lanes are linked with the upcoming Ivy Bridge CPUs, you will have to upgrade your motherboard’s BIOS to recognize and communicate the new standard without issues.
Most current motherboards only feature PCI-E 2.0 switches (a notable one being NVIDIA’s NF200 for “True SLI”), so when an Ivy Bridge CPU that has PCI-E 3 capabilities is plugged in, only eight lanes (out of the full 16) will actually function.
Motherboards with multiple PCI Express slots have the most up-to-date switch chips from vendors like Pericom or PLX to actually support the full Gen 3 bandwidth availability on NVIDIA SLI and/or AMD’s CrossFireX multi-GPU configurations. Many older motherboards that claim they are “Gen 3” ready are just hiding behind the fact that they are compatible with Ivy Bridge CPUs and aim to take advantage of consumers uneducated on the technical aspects behind this technology.