I'm looking at getting a motherboard with PCIe 3.0 slots. I don't have a graphics card that has 3.0 compatibility, so I don't care if I'll be able to utilize 3.0. What I am worried about is if I'll be able to use the 3.0 slots at all with a Sandy Bridge processor. Can you use PCIe 3.0 enabled slots at 2.0x16 without an Ivy Bridge processor?
With a single PCI-E 16x slot and running a 2nd generation Intel® Core™ processor the slot will run at 2.0 speeds. If you are going to run 2 video card on the board the slots will drop to running at PCI-E 8x/8x at 2.0 speeds in most cases (some boards have a 16x/4x step up). The only way to get faster speeds from a board that does support PCI-E 3.0 is to put a 3rd generation Intel Core processor in.
Current graphic cards are not bottlenecked by PCI-e 2.0 Probably next very high end graphic cards ($500+) may be start to be a little bit bottlenecks. Nothing to be bothered at all. But in 2014 I would think that the very high end performance graphic cards will be bottlenecked by PCI-e 2.0. I would guess by 10%, maybe 15%.
If you do not buy very high end graphic cards, then you really do not need to be worried at all. A $350 graphic card probably will not reach the level of a 10% - 15% bottleneck in a PCI-e slot for another 4 years. I'm pretty sure by then you would have saved up enough money to drop an Ivy Bridge CPU in your mobo. Or you would have completely upgraded your PC to the latest and greatest by then.