With a few exeptions the pci-e slots are all backwards compatible. Sometimes a 2.1 card can give some problem with an old 1.0 mobo, but that's rare.
Also a 2.0 or 2.1 card should work in a 3.0 slot i think.( tho not complete sure about that )
Since you're still building i would wait maybe for the 7xxx series from amd. ( if you can wait for a couple of months that is )
Don't think I can haha. Everyone I know is playing BF3 right now and I don't wanna show up too late to the party. Best I can do is wait 'til Black Friday when I'm hoping a nice i5/Mobo deal will pop up.
Gonna do some more reading up on everything, I've never overclocked a card or built a computer before so this is all new.
Thanks for your help.