polod :
need help deciding for my graphics card..
Comparison
The GTX 660 Ti:
+ Consistently performs ~10-15% faster somewhat consistently.
+ Capable of fully utilizing GPU-based PhysX in any current form. (Physics effects in games).
+ Capable of playing with TXAA enabled. (New form of anti-aliasing).
+ Usually quieter on full load.
+ Generally lower power consumption
The Radeon 7870
+ ~ $74 cheaper
+ Temperatures are generally better (Cooler)
+ In compute benchmarks, the Radeon 7870 is consistently better by an extreme amount.
Conclusion
If it's for gaming, the GTX 660 Ti is somewhat consistently faster than the Radeon 7870. If you're going for pure performance and don't mind the increase in price, you should probably go for that. If you want something cheaper for its performance and don't need that extra raw power, then the Radeon 7870 is an excellent choice.
Alternate Recommendation
If you were to go with the GTX 660 Ti, I advise you to reconsider for a GTX 760 instead on the Nvidia side. The GTX 760 provides consistently better performance than the GTX 660 Ti, and is generally about the same price. As for a GTX 760 vs Radeon 7870 comparison, just look at what I said previously about the GTX 660 Ti, and consider a small-moderate jump in performance with the GTX 760.
Here's a link for the current cheapest on Newegg ($256):
http://www.newegg.com/Product/Product.aspx?Item=N82E16814127744
I hope this helps