I have the 650ti boost. In a few benchmarks I've seen, it runs slightly faster than the 7850. I have an Asus version which runs very cool. It held up when I was playing the BF4 beta on 1600x900 med settings. I'd probably just go with the 650 if you want gaming performance and it has physx support. Get the 7850 if you need more compute for GPU accelerated programs.
And you can find cheaper versions of either card depending on the features. The best price/performance is the cheapest 650ti boost but it might not have all you want, and if only $10 separates what you want from it, go with the one you want.
Check these:
http://pcpartpicker.com/parts/video-card/#c=132,81&sort=a5&qq=1
Tom's guide:
http://www.tomshardware.com/reviews/gaming-graphics-card-review,3107-3.html
*Just looked, my card is the $124 Asus one... it was $160 just a month ago, I wish I was in your position. It has a 6% off code that expires today, I'd recommend it because I actually have one!*
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