Rather than a M4A79T i would suggest a M4A89GTD using the 890GX chipset, it supports 8x/8x crossfire which barely impacted two 5850s in their initial review so its effect on 5770s would be negligible, its cheaper than the M4A79T and it has some new nice features like SATA III and USB 3.0 which will be useful soon. http://www.newegg.com/Product/Product.aspx?Item=N82E16813131631
1600MHz being listed as (O.C.) just means that it wont automatically read the SPD settings for 1600MHz ram and will set it to the defaults for 1333MHz memory unless you go in and adjust it which you should be doing anyway.