The problem is what "may" come.... Till Madshi introduced Jinc3 + AR there was nothing that a 550Ti could not handle and it was cheap, quiet, low power etc etc. Nice Card, but the 50i stuff was around 90% and just a little additional push on 60i broke playback so I'd agree that running a card close to the top does not give you much overhead.... and then madshi will introduce AR+ and you would be buying a new card again. My PS in the Shuttle s a "80 Plus 300W" that runs not only the 660 (only requires 1 power cable), but an i7, SSD, and BD Drive all fine. I'm not has happy with the 660 as I was the 550 as it is noisier, bigger (real cram to get it into the case - bits pushing on each other etc) but .... the 550Ti was just not powerful enough - compromises have to be made!
Also if you don't play any 60i stuff then .... who cares or if there was a way where you could pick and choose what madVR to apply by content specs (else you could just turn off the AR for this stuff ...) you might be good.
You might want to PM SamuriHL on this thread as he recons he got a 550Ti playing 60i stuff without maxing out.