2 questions :
a) will HD79xx series Radeons (PCI 3.0 cards) work in older motherboards which are only PCI 2.x ?
b) assuming that it works but is limited to 5 GT/s (instead of 8 GT/s), would there be any appreciable performance difference in games @ 1920x1200 between a 7970 and a 6970 ? Or would Nvidia 580 be a better option (particularly if prices reduce in a few weeks) ?
a) Yes, it's compatible.
b) if it's running at 16x speed, you will be fine. And unless I'm misreading your question, of course there will be a difference between a 7970 and a 6970 at any resolution, because the 7970 is a much faster card. I'd say wait for Kepler, though, just to see how it stacks up against the 7970. Judging from nvidia's claim of running samaritan on a single kepler gpu, it should be pretty sick.
Thanks for the info. Question (b) was more "how much of the 7970s performance advantage over 6970 is simply due to it running in a bus with faster interconnects, as opposed to it being a more efficient architecture ?"