If you're playing at 1680x1050, it probably isn't. At 2560x1600, it's an insane difference. The 5870 is equivalent to a GTX295 in performance, and is much smoother (has much higher minimum framerates). The 5870 really is a high resolution geared card.This is why I'm skeptical of buying a 5870. I don't know who is benchmarking what on other sites, but real world, it doesn't seem like such a big step up. But, op, even if your 280 benchies look low.
OP, try Driver Sweeper and a clean reinstall of the latest beta Catalyst drivers. Also, neither of those games is very stressful, so you probably wouldn't see much of a difference. Try some more taxing games (Crysis, STALKER: CS, Far Cry 2, etc.).