Welcome to SPCR. Tech Report
sums it up nicely with:
In the end, then, the R7 260X's only real advantages over the 7790 are its higher clock speeds and its larger frame buffer. The two cards have otherwise very similar specifications...
..Those similarities are compounded by the fact that many, if not most, retail 7790s are clocked higher than the reference speeds you see above. AMD bragged about this fact earlier this year. It's not uncommon to see retail 7790 cards with peak GPU and memory speeds of 1075MHz and 6.4Gbps, respectively—quite close to the reference R7 260X.
The R7's larger frame buffer is a notable improvement, though. Variants of the 7790 cards with 2GB of memory do exist, but they're few and far between. Newegg carries only two of them right now: one priced at $139.99, and another at $169.99. The R7 260X will make 2GB of video RAM the new standard at $139.99. Given the slew of graphically intensive next-gen games coming later this fall, that's probably a good thing.