We also need to know what resolution you will be gaming at, otherwise you're going to be throwing money away for a sexier card that will get you little to no performance boost.
I found this article for you, you should take a look. Its a review of the ATI 4890, but in its testing it matches up a number of different cards including the 4870 (what I have, a great value card IMO), the 4870x2, GTX 285, and GTX 260.
