I'd probably hold back for now, I mean, the 4870 isn't exactly a bad card, in fact, it's pretty awesome. I'd keep it, do you really need a better card, are there games you can't play?
If you save that $200 and then whatever else in the future, when a new card comes out, jump all over that. At least that way it'd probably be a worth while purchase.
I was thinking of selling my 8800's and upgrading to a GTX260 or something like it, I had to stop myself and think about it. My 8800's are probably about the same as GTX280, so there wouldn't be any point. They're older cards, but they keep up with the newer ones, they just seem 'old'.
Hang on to it, man!