In the review of the launch of the 5770, they test a 4770 crossfire.
Also TH did a good article on 4770 crossfire unbeatable at 220.http://www.tomshardware.com/reviews/radeon-hd-5770,2446-12.htmlhttp://www.tomshardware.com/reviews/radeon-4770-crossfire,2288.html
4770 handles a 5850 up to 1920X resolution, not beyond because of the 512 memory.
The only negative is NO dx11, I'm in the same boat as you. I might do it. I only game at 1680x right now. I can o/c 20%, which gives me pretty awesome frame rates in almost everything. But I would probably enjoy even more FPS and IQ !
edit: If I was you , I would go 5850 or 4770 crossfire. I don't think the 5770 is enough of a upgrade for 160.00? Unless you have a buyer
for your old card, right away.