sounds like you have done a few bit of research already.
Yep Asus got a great line up products for different price range.
Standard z77 board would work also. There is not much difference apart from a few more pcie lan and extra wifi, slightly better cooling?? and few extra port you won't usehttp://www.newegg.com/Product/Product.aspx?Item=N82E16813131820
6gb ram sounds highly irregular, normally they comes in either 4gb or 8gb for optimal performance.
650watt is fine.
I think the i5 3570k is the standard recommandation for gaming pc just like the 2500k was