If you can afford a 7950 then get it, it will out perform any of these cards. The next best would go to the 660 Ti in my opinion. The 7870 XT and the 660 Ti perform so close but if you look at power consumption the 7870 XT is like a GTX 480 v2.0. By the way I only recommend the 660 Ti if you can find it on sale for around $260 or less which you usually can at NCIX. If you can't then just get a 7870 XT but make sure you have a good PSU, a quality 550W unit would be great.
EDIT: Okay, maybe not GTX 480 v2.0 but it does tend to use more power than a 7950.