I'd probably just get a 5850 for the price of two 5770's and I would definitely recommend that over the GTX 470. NVidia used to make better cards, but ATI has passed them up in performance these days, in my opinion.
Is performance and value the only reason for the 2 5770 in CF what about "anti-bottlnecking", image quality, and future proofing are these factors in ATI's favour also is the HD 5000 compatible with 120hz displays for the future?
I'd have to give ATI the edge right now in most price segments but the two which I don't are the GTS 250 and the 470. The 470 substantially outperforms the 5850 in just about all DX11 tests at 1920 x 1200/1080 and hi settings. Older Dx10 games like Crysis, the 5850 has the edge.
My thinking is two years from now, the 470 will still have value if you add a 2nd one....not so for the 5770. I'd get the 5850 or the 470 .... two 5850's outperform a 5970 .... twin 470's substantially outperform a 5970, but you'll want an 850 watt PSU when you take that step.
The 120 hz displays are here and both ATI and nVidia can display on them.....only nVidia can do the 3D Vision thing on them tho.