Sorry but from the 7870 upwards AMD totally beats Nvidia at every price point simply because the Tahiti chip overclocks so well.
If you're on a budget the 7870 LE/XT will give you fantastic performance for the money.
If you want something better, the 7950 matches the 660 Ti but costs less.
And then if you overclock the 7950 it easily becomes a 7970 Ghz which competes with the 680...
edit: Also, if you are talking high resolutions the AMD cards scale better. And any AMD 7800/7900 series card will get you two games for free... Have fun