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If you've long been distraught that there isn't a modern card that sits between the GeForce GTX 650 Ti and the Radeon HD 7850 in both price and performance, then the Radeon HD 7790 is just the card for you. The rest of you can carry on, as there isn't much to see here.
Basically, it's a new GPU with 14 GCN CUs (as compared to 10 in Cape Verde and 20 in Pitcairn), and a 128-bit memory bus clocked higher than earlier Radeon HD 7000 series cards so as not to immediately hamstring the card for lack of memory bandwidth.
The interesting thing about it is that, while the 7790 is a lot faster than the Radeon HD 7770, it barely uses more power. That makes the new 7790 the most energy-efficient desktop video card yet. In a desktop, that's not a big deal, but it could make for a nifty part for laptops. I'd attribute the energy-efficiency gains to AMD having the chance to both go back and make little tweaks to the architecture here and there, and also launching it on a more mature and better understood process node.
Alas, all that AMD has new on the discrete GPU front this year is this card, perhaps a salvage part of it eventually, and a lower end chip to basically replace the Turks GPU in the Radeon HD 6670 and some mobile parts.