HiTechObsessed :
http://pcpartpicker.com/part/xfx-video-card-fx777aznj4
Will be the best performance for the money. Better than the 250 and 650.
Just a comparison of the R7 260X and the 7770:
http://gpuboss.com/gpus/Radeon-R7-260X-vs-Radeon-HD-7770
And the pricing:
http://pcpartpicker.com/part/gigabyte-video-card-gvr726xoc1gd
They are both the best cards in that pricerange, get whichever is cheapest.
Second this, as long as it's not a CX 430 or CX 500
Well, a CX 430 would be better than a lot in its pricerange. You're absolutely right though. Their capacitors are rated for low ambient temps, and since this build would be using a lot of a 430's wattage, you might run into problems. Here's a review of the 430 all the same: http://www.jonnyguru.com/modules.php?name=NDReviews&op=Story&reid=214