4870>>HD5770/GTX260.... Anyway. HD 5850 is the smartest deal in my opinion...
again we have to say this on the forum, those benchmarks are from the beta driver release of the 5770, and even if the 4870 is better by a small margin (still), don't use ">>" that implies much greater
@ Ebkboy: As the others have said: Stick with your dual GTX260s'.
In point of fact, most benchmarks show the HD5770 as being slightly slower than the previous generation cards in its performance range (HD4870/GTX260) so you'll most likely see a performance drop off along with the loss of PhysX. The only reasons to change is: Lower power consumption and, possibly less noise (my reference 5770 is as quiet as a meek church mouse, even under heavy load).
It wouldn't be a big improvement. In some situation you would loose a little performance. As previously mentioned you would also loose PhysX (though that may not be much of a factor in the future). Anyway, you should wait till DX11 is more prevalent and by then the price of two 5850s, or their replacement, should be alot less.
a single 5870 wouldn't be an upgrade, just a sideways movement
for upgrade he would need: 2x5850,5970,2x5870 for it to be worth it
but each of these solutions are $600+, not to mention probably a new PSU
I agree with everyone else it's a sideways move in terms of performance, especially if you have a hefty enough CPU. If you could make the move from SLI to Crossfire without a mobo swap, then likely you do have such a CPU.
As you can see the 5770 is almost exactly the same as the GTX 260 192 core and the 4870 in performance. The GTX 260 216 core is faster by a good 10%.
So either you will get the same performance, or you will get less. If DX11 is really important to you then I recommend waiting for the GTX 4xx series launch in March to see if you can get 2 5850s because they should drop in price. On the whole I don't believe anything less than 2 5850s will be a worthwhile upgrade.