You will not get 30% increase in fps with overclocking GPU. But ofcourse you can overclock if you want to, but get a better PSU, something like 750W or 850W. You can easily overclock your CPU (to around 4.5Ghz) and GPU, afterall Matrix was made for overclocking, running it stock is kind a "missing the point", and if nothing else, you payed extra for all those overclocking features - use them.
In the end you will have few fps more, expect around 10% gain if you manage to oveclock GPU over 1200 Mhz.
P.S. Memory of 7970 Matrix is running at 6600 Mhz (not 6000).
