You normally overclock either or both of them: depends on where your "bottle neck" is. My own opinion is that you don't overclock either of them. If they can safely operate faster, the manufacturer would already have them doing that.
Can you overclock? Sure.....lots of folks do. But overclocking raises temps and failure rates. And when either fails from that overclocking, catastrophic failures to other components occur.
Be happy with what you have a save up for something better.