I'm building my first computer and I have all of the parts pretty much picked out except for a graphic card. I want to use the computer mainly for gaming and stream it through my tv. I have a 52" LCD tv.
Assuming you are willing to spend $250, as a single card the reference 6950 beats the reference 560 by 5% but if you pair them in SLI / CF, the 560 wins by 5%. However, the factory overclocked 560 sells for the same price as the reference 560 thereby erasing the single card win of the 6950. OTOH, some models of the 6950 can be flashed to a 6970 but that voids your warranty.... and no OC'ing does not void ya warranty unless ya start messing w/ voltage.
At $350, I'd get a 6970 or 570 .... though the OC'd 560 falls just short of this performance level. Neither of these cards overclocks well. Again, the 6070 ekes out a slight win 526 fps to 524 in the single card competition but the 570's in SLI win big with an 873 to 825 spread.
You can get away with a good 650 watter (Corsair HX or TX, XFX anything, Antec SG / TP New) for the reference 560's , an XFX Black would be the ideal choice for OC'd 560's and an 850 watter (Antec SG or CP, Corsair HX or TX, XFX Black or Core) for the 570s / 6970's. If overclocking the CPU also, Id saty with the XFX Black, Corsair HX, Seasonic X series or Antec SG or CP series.