a single 460 will outperform a 5770 but it won't against x2 5770's.
I don't know how to unlock cores but i think it has something to do with the mobo,you should look into that when selecting the mobo.If it says 6 channels than it's a 6.1 which is surround sound,so it's capable.If you aren't going to buy anything else i would get some spearks to go with it,
Your mobo is very good
Supports Exclusive ASRock UCC Chip - Unlock CPU Core feature
^i guess you can unlock the 4th core with that mobo
and
7.1 CH HD Audio with Content Protection (Realtek ALC892 Audio Codec) , Premium Blu-ray audio support
So that means you can hook up a 7.1 stero system to it something to think about if you have the extra money and you want to spend it on something,i can't think of anything else you would need,Can you?