The Vapor-X cooling solution does allow for increased overclocking margins to a point where the core is only going to give up what it can. Voltage tuning helps, but you reach a point where voltage no longer helps drive a core clock speed increase. This particular HD 7970 6GB Vapor-X GHz Edition sample was able to reach in excess of 1200MHz on the core and 1700MHz+ in some tests but would fail at others. The final clock speeds of 1175MHz on the core and 1661MHz are the levels that would pass all of the test suite along with a series of gaming sessions without failing. The core speed increases of just over 17% on the core and just over 11% on the 6GB of GDDR5 memory are enough to drive measurable performance increases."
Slow computer got you down? Maybe a hardware upgrade should be in your cards, if so this guide to the Fundamentals of Upgrading a PC is an invaluable read. PCSTATS Tips