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.