Probably not....if you go by these benchmarks, it's a only 8 frames more, in favor of the 2.8. Not worth the extra cash, IMO especially if you are only seeking the extra horsepower for gaming.
Those benchmarks just look at one older game, Halo. With modern games, the Radeon HD2600 will reach its limit before the 2.4Ghz Core 2 Duo, so the 2.8 won't give any advantage.
I never said I wanted a "gaming Graphics card", although I would be ecstatic to have one in an iMac. Hell, at this point I'd rather have the 7600GT in there over the card that's in there now. FYI - the 7600GT was, and still is considered a "gaming Graphics card" and was in the last model iMac.