The 550 should perform a few FPS slower than the 6850 but I suggest waiting for 7800's to launch. This will force both AMD's old cards and Nvidia's near the price range down a far bit.
the only official comment on AMD`s website says a release date of jan 9 only for the 7970,release dates for all others would be who knows when.
no info on any other 7000 series release dates announced by AMD.
``The AMD Radeon HD 7970 is available January 9, 2012 from retailers worldwide, with select models starting at $549 US SEP``