Way I see it, these cards below are about equals so:
6970 = 570 in performance, so which ever is cheaper is the better buy. (obs! 6950->6970)
5870 = 560 in performance, so which ever is cheaper is the better buy.
6850 = 460 1gb in performance, so which ever is cheaper is the better buy.
5770 = 550 in performance, so which ever is cheaper is the better buy.
more or less, but.no, its a bit more complicated than that afaik.
6970 > 570 . It has more or less the same benchmarks with a tiny advantage, uses less power, and has more vRAM, making a decisive point in the 1920+ resolutions and more future proofing.
Also the 6950 without the unlock (which is bound to exist sometime soon) > 560 in performance and is at comparable prices.
Also you forget 6990>590 and 6990 cheaper and better buy.
Overall, if one thinks price/ performance, the whole picture is red, with a tiny green stripe @ 500 bucks with the gtx580, and another tiny green stripe @2.000 bucks with the quad SLI gtx580.
Oops my mistake, there is no green stripe before 2000 bucks, and that's because you cannot SLI / CF beyond 4 gpus.
In fact the green stripe @ 500 bucks exists only if you think single card, my bad.
As an addition, some people like green better, some prefer red, and some people actually care about features like eyefinity or nvidia 3d (not talking about surround till they reach eyefinity specs).