I have a Sapphire 7950 OC dual fan and I couldn't recommend it highly enough. Now, you can get the 950Mhz version (mine is 900) and it comes on a nicer PCB (black instead of blue) which is actually used on their flagship card, the 7970.
In terms of overclocking, it also has massive potential. You should be able to hit 1100Mhz no sweat with the 7950 as long as you have the appropriate air flow. The 670, in my opinion, seems a little more tricky to OC due to the boost clock. So if you have never overclocked before but would like to start, the 7950 may be the best all round card for you. I have mine running faster than a stock GTX 680 here.
If you are impartial to brand, the 7950 is an excellent choice. If your heart is set on Nvidia, I would save a little extra for the 670. So many great benefits over the 570 for a little extra cash, you will thank yourself later when Crysis 3 and the new Frostbite 2 engine games drop.