Sandy Bridge-E has supported PCIe 3.0 from the beginning, it's not a driver thing.
Some motherboards do and some don't and some of the earlier stepping CPUs also had problems. When SB-E was released PCI-E 3.0 was meant to be supported but in practice there were no GPUs to test with, and when it was released PCI-E 3.0 was not on the support list. The motherboards did support it but once the cards actually came out some didn't meet the specification precisely and it was a hardware problem.
If you buy a modern motherboard and CPU then it should support PCI-E 3.0, if you bought when it came out it might support it, and it might appear to support it but actually not do so perfectly causing crashes and freezes.
yes but, NV drivers would make the card work in 2.0, unless you modify something,
Nvidia's cards don't like the timing problems of the early SB-E's and since the extra bandwidth makes no practical performance difference they disable it on all SB-E's. Its easy enough to modify the registry and bring it back, but you will want to be quite sure that the machine does indeed support it, not just that it says it does in the bios.
My 7970's worked at 3.0 but my 2x 680's do not, they crash every few hours. Seems to me my Asus X79 pro and/or 3930K (first stepping) don't support it fully even though the 7970s doesn't have a problem.