I don't want to buy any more RAM right now, so how can I make the best of the RAM that I have?

I have 2 x 4 GB @1600 MHz and 2 x 2GB @1330 MHz Both are Corsair.
Each pair seems to be running dual channel, but I think they both run at 1330 Mhz

Is it possible to run both of them at 1600 MHz, how would I go about that?

My motherboard is Asus P8P67 Pro.

Any tips appreciated!