7970 and 550w psu??

Once you start overclocking its very hard to tell exactly how much power you would end up using. My guess is you have this 550W which should be enough to handle your current needs. If you wanted to be safe I'd normally go at least 100 watts over the minimum which is 500 watts suggested with the 7970.
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

You should be able to go for a moderate overclock, depending on your CPU TDP.

Right he may be able to make a moderate overclock nothing crazy however at this point the 7970 at stock is way more then enough on a single screen configuration. Maybe in a year or so to squeeze more toothpaste out he would want to overclock.