If you were to trust the "leaks" of the kepler series, kepler would be the victor with at least a 20% performance margin
Thankfully, we all know these "leaked" benchmarks are bs (Though I honestly hope not). If you are really thinking about buying kepler, it should be released sometime later this month, nvidia has to answer the 7000s soon. From what I can see, the only adavantage the only advantage the 7870 has over a similar card like the 570 is less power consumption (im also going to assume cooler performance) and of course, a higher price
(with on average of 5% better performance than the gtx 570). In all honestly, keep your card until you see the lineup that kepler has to offer, so at least you can make an informed descision and actually SEE what your options are. AND if you don't like the 7000 or 600's, you can always buy yourself a 580 for dirt cheap after the 680 comes out