I wouldn't pair a 7970 with a 450 watt power supply that's more dangerous than keeping his current psu. if it's ocz then they're a good manufacturer, not the best but it does as it's intended to do very well, if it's a 600 watt power supply it should have at least 1 x 8 pin connector normally it's a 6 pin with a little 2 pin hanging on the side? if not then I don't like using adapters though it can be done it isn't the best solution I'd look at getting something that takes 2 x 6 pins which even the 770 doesn't unfortunately, the only card which does take 2 x 6 pins would be a GTX 680 which can still pack a punch in todays games despite the age of the card! coming upto 4 years old already I believe?

if you're happy using a connector however then grab a 7970 like I said though a 600 watt psu should have an 8 pin and a 6 already