If I were you I'd go for one Gigabyte 1GHz Super Overclock Edition GTX560Ti. When it comes to GPUs, this baby performs like a 570 at a $270 price point. Probably the best bang for the buck right now on the market.
Not much point in factory overclocks when you can just overclock cards yourself and almost always to higher clocks.
But if you want to keep your warranties you may want to. I personally just go for the cheapest version (albeit to some choices on companies), and will only go for an OC if they're cheaper, the same price, or only a tad dearer.
But the 560 is better in a few cases. It's hard to find direct comparisons as the benchmark suites have all changed. But I would say that considering CF occasionally has issues, while a single card is basically always going to work, it might be best to just buy the 560.
They are comparable in performance assuming you are playing a game with CF support.
The 560 (and by extension the HD 6950) is a much better choice because you can SLI it later down the line and single card solutions will have less problems and better support than SLI and CF. A single 560 will produce less heat and consume slightly less power than the dual 5770s.
Rule of thumb: when performance and price are the same, go with the single card.
I agree with wolfram 5770 x-fire is a 5870 or beyond, of course when crossfire is not being utilized then your stuck with a 5770 but many problems of the past have been worked out in newer card series.
Also the gtx 560, especially when oc'ed will be equal to or better then a 5870... it just depends you have a higher potential performance from 5770 crossfire I'd say but the gtx 560 will likely be a bit more reliable as a single card... it a tough call I'd say get the 5770s if you find a stellar deal otherwise the 560 is the logical choice...
the only reason you should be buying 5770s is bc of the price, if you want that level of performance why not buy a single 5870? those are also super cheap now so its a tough call but there are some thoughts for you to ponder
also if you went with the single 5870 then you could add another a bit down the road (if you have the psu for it) maybe for even cheaper: