It should be enough, i mean you might loose the luxury of AA after a while, but I see even 2 8800 GTs doing well @ that resolution:)
As texture become more complicated and such, it might cripple your results though.
I wouldn't suggest going from a 4870 x's 2 setup to a single version of it just for extra ram, I would wait it out.
Unless your a millionaire and poop money:D then send some this way:P
I would wait and see what the new generation will dish out in a couple of months (6 or so).
I mean you'll be able to do 4 x's AA in a lot of games (not Crysis) I would look at reviews and see wherethis will leave u:)