It appears that you can only run a GPU with 1 x 6 Pin. The fastest 1 x 6 Pin card right now is the Radeon R9 270. It is slightly faster than the GTX 660, which is Nvidia's fastest. Soon the GTX 960 will have this crown and, given the efficiency of Maxwell, it will wear the crown quite fashionably as it is expected trump the R9 270 in performance by a significant amount. But before you trumpet the coronation horns, keep in mind it will be out of your price range by nearly double. In that sense, there's no need to wait. Buy now.
The R9 270 can be found for $130 easily if you take into account rebates.
http://pcpartpicker.com/parts/video-card/#c=154&sort=a8
Let there be no room for misinterpretation: the R9 270 is faster than the 750Ti in most games. Only occassional exceptions like AC Unity give the GTX 750 Ti any favourable light. While the GTX 660 is more likely to trade blows with the R9 270 from time to time, it still loses on average and Nvidia has not been optimizing for Kepler as well as Maxwell or AMD for GCN - in other words, it's not likely to make gains on the 270.
If rebates are out of the question, and you are willing to budge on a paltry $5, then the R7 265 is the best answer:
http://pcpartpicker.com/part/xfx-video-card-r7265acnj4
It still beats the 750 Ti most of the time.
If your $130 is absolutely firm (no rebates, $5 bust is too much) then it comes down to 260X vs 750 Ti. I would easily take the 750 Ti in this case since it is cooler, uses less power, and is probably the faster card overall. But I recommend either using rebates or going over budget $5, since the 265 and 270 are simply faster cards.
750 Ti, R7 260X, R7 265, R9 270/270X and GTX 660 are all in the same ballpark of performance. Maybe 25% difference there bottom to top, but it depends a bit on which game you are playing which will do best (except between the Radeons).
There is too much variance to group all of these together. The 750Ti and 260X are simply slower and a full performance tier below the 270/GTX 660/265.