2 HD5770's would be equal to a 5850 from what i've read online and the GTX460 1GB is an equal competitor to the HD5850 imho. It would be cheaper to go 1 GTX460 1GB than 2 HD5770 1GBs.
Because your going with an amd system I would go for the future crossfire setup since you can't sli (damn you nvidia!)
I would also go with another motherboard, one thats based on the 890FX chipset instead so u can get x16/x16 unless your not worried about that. Here are my 2 suggestions for you...
Newegg.com - GIGABYTE GA-890FXA-UD5 AM3 AMD 890FX SATA 6Gb/s USB 3.0 ATX AMD Motherboard Newegg.com - ASUS M4A89TD PRO/USB3 AM3 AMD 890FX SATA 6Gb/s USB 3.0 ATX AMD Motherboard
Good luck with you build