It is based on a cut-down version of the "Barts" GPU first seen in the Radeon HD 6800 series and as such it shares features like improved tessellation performance, Eyefinity enhancements, and the ability to accelerate Blu-ray 3D video over HDMI. AMD's press materials for the 6790 mention a TDP of 150W and two 6-pin PCIe power connectors, though board partners will be shipping their own designs and some will require a single power connector, which is probably a good thing for a card in this price range. Most cards from board partners will also ship with a custom cooler.
Overall performance is indeed above the GeForce GTX 550 Ti, according to reviews. Then again, the latter can already be found for around $130 or less while the more powerful Radeon HD 6850 is available at $165 before rebates. With such a small difference it's hard to tell how relevant this card will be unless street prices come down a bit. You can check out some reviews at Tech Report, Tom's Hardware and AnandTech.