Ah! now you don't have to link us your PSU model, I found it! So it's Cooler Master Extreme 2 625W? http://www.coolermaster-usa.com/product.php?product_id=3083
If yes I wouldn't recommend you overclocking on this PSU it can fail in your system and also just for note when you shop for PSU avoid cheap PSU's and refer models in this list http://www.eggxpert.com/forums/thread/323050.aspx
You can try to overclock your GPU but it's on your own risk! http://www.theregister.co.uk/2012/11/26/exploding_computer_vs_reg_reader/