Of course you can use it (it's not REALLY 3.0 anyway without an Ivy CPU, but even if it was, it would still work perfectly). And it's capable of PCIe 3.0 with an Ivy CPU, BUT, it's not even worth worrying about. PCIe 3.0 only matters when you go Tri and Quad SLI/CF, and even then, it's not much of a difference. PCIe 3.0 is just window dressing for now and will be for a while.
Well, with that mobo, you'll HAVE to get the 2500K, unless you have another Sandy CPU laying around to flash the BIOS with because the 3570K won't work without a BIOS update on Z68 boards.
If you want to go with the 3570K, I'd suggest a Z77 board that will work with it straight out of the box.
As far as which CPU is more "future proof" they'll both last as long you as long as you need them to (they'll both be struggling a bit in 4-5 years), but the 3570K is slightly faster (5-10%) so that's the one I'd go with, with a Z77 board.