Well the 6850 you linked's core clock sure doesnt beat 550ti's core clock. Idk i know its a hard decision on a video card (im also looking for a new video card)Figured a 550ti might work nicely.
Core clock isnt everything. The 550Ti honestly.....sucks. I would rather have a 6770 than one of those.
The 6850 is also much faster than the 550Ti. At 550Ti price, there are much better options.
Also all of the GPU's we've been discussing should fit in the case.