Can't decide, my build would be used for gaming and a bit of linux programing.
Radeon 7870 is a better performer for gaming, if you can find a 7870XT then pickup that one , because that card is diffrent from a normal 7870, the XT is based on the tahitie Le chip same as the 7950, its basicly a 7930.
Eh, it's 40 bucks more expensive so I'm gonna pass.
I've used both and the 7870 is far superior.
A GTX 660 is only slightly better than a Radeon 7850. A Radeon 7870 is much better and would be more comparable to a GTX 660 Ti.
http://www.anandtech.com/bench/Product/548?vs=660
http://www.videocardbenchmark.net/high_end_gpus.html I like my 7870 . i havent even oc it yet