Yes PCIe is backwards and forwards compatible. It will run at PCIe 3 speeds on a PCIe 3 slot. This will result in a small (1-3%) performance drop but nothing worth building a whole new PC for.
Oh alright, so PCIe 3 isn't saturated by these cards.We won't know until reviews and benchmarks hit but for what it's worth, on the graphs they showed during the presentation, they used an Intel CPU so they had to be using PCIE 3.0 since Intel doesn't support 4.0.