I'd keep it if you don't use any AA and only play crysis. If you plan on upgrading to a bigger monitor soon, I'd exchange it for the 4870x2.
The GTX 280 is a great card, don't get me wrong, but the 4870x2 is the stronger of the two, and the microstuttering some people mentioned(people who have never even used the card to begin with) is not noticeable at all on the x2. I've actually used both during my last job, granted I never got to spend much time with either, but at least I've tried 'em both and dug the 4870x2 more.
It's also cool to know that you have the fastest card on the market!
(Nvidia's drivers are still way better though, but that might be because I'm used to the nvidia control panel.)