I would recommend the 670 only if you don't want to overclock because it beats the 7950 at stock. But when overclocked the 7950 will beat the 670 overclocked to it's max. So do you plan on overclocking?
Im not really sure about overclocking.
how hot will the 7950 get when overclocked to its max on full load?
How much betterwould it get?