2 x 5770 is on par with a single 5870, and it would have DX11 support (despite what you say, as they are DX11 cards as well)
@OP: between the 5850 and GTX470, its up to what brand you prefer, 5850 can be had for $300 and the GTX470 for $350, 5850 uses less power but is a tad slower than the GTX470
Ah, you're right about DX11. I read that the 58xx series was their first to have it, but apparently 57xx came after. Did not know that.
I have not looked into crossfire 5770 very much so I don't know that it is comparable to a 5870, but if so, then that's definitely the more cost effective route to take. Otherwise probably go with the GTX470 since it is still in your budget, but slightly better than a 5850.