Dunno why your post is so big, but yes, it'll work fine - PCI express is backwards compatible, and you shouldn't get more than 2-5 frames less than you would.
DarkSable means that you're only going to lose 2-5 FPS. If you would get 60 FPS using a PCI-E 3.0 card in a PCI-E 3.0 slot, then putting a PCI-E 3.0 card in a PCI-E 2.0 slot would get you between 55 and 58 FPS. Seriously, I doubt you'd even notice the difference except in benchmark stress test software and the highest level cards.
^Exactly. You'd get basically the same fps, is what I was saying.
There's really not much difference at all - pci 3.0 lets more information go through at once, but since most cards can't saturate a pci 2.0 bus, it's a moot point.