Stranger, we might be talking about two different things here. Some people say "big deal, you got 30FPS and I got 20FPS, so I'm only 10FPS slower." The problem is this is absolute values, and totally ignores percentage. His "not bad" 20FPS is actually 33% slower, which sounds a lot worse, because at these low FPS it IS worse. Same is true if you are talking about 40FPS instead of 50FPS. Might be only 10 frames, but its also 20% slower.
If the 4970 is going to be 30% faster then the 4870, then we know it will come close to the GTX280. You can't just say it will add 10 FPS to your games, as that lacks any real detail. (what res is the first that comes to mind.) By saying 10% slower then the GTX280, but 30% cheaper, we have some idea about its performance.