The 460 is better than the 5770 but it uses more power,i think it would be best to go with a 5770 anything more than a 460/5770 and you would start to get a bottleneck,that is a very good PSU so i don't think you need to worry about it.
It is up to you but, i would reccomend that you buy a 5770 and O.C. your CPU to 3.0Ghz and get 2G more of Ram.
5770 $140=$130 After MIRhttp://www.newegg.com/Product/Product.aspx?Item=N82E16814141116&cm_re=5770-_-14-141-116-_-Product
You on your own for the memory(Ram)but i would suggest you get some.Make sure it's the same type as one you have in your PC right now,best way to be safe aobut is to get the exact same stick thats in there now.