There is not such thing as a future proof PC, yet there are parts that can be long lasting to an extent.
What Jeol said was correct, however most motherboards have USB 2.0 ports around 4 compared to the 2 USB 3.0 ports when purchasing a lower budget motherboard.
Now I will give you a few builds (because I don't know what manufacturer you prefer) that would be around your budget (or less):
Intel: http://pcpartpicker.com/p/rWTqP6
Intel + NVIDIA: http://pcpartpicker.com/p/f4QxRB
Intel + AMD: http://pcpartpicker.com/p/3jDTpg
AMD: http://pcpartpicker.com/p/nnBWzy
AMD + NVIDIA: http://pcpartpicker.com/p/6824YJ
AMD + AMD: http://pcpartpicker.com/p/kDsxRB
I know that the A10-7850K supports 2133MHz RAM, but there was none in a single 8GB stick to save costs so I went with the same RAM that the 760K uses.
So these are your builds. Now as you can see you can just get yourself an Intel build without a GPU and save up some more money to afford either the 750 Ti or 265. And if you like something that makes you feel more "future proof", you can upgrade the CPU to a higher class CPU (though I do recommend getting a better cooler if you do that).
Now the AMD side is a lot cheaper when you throw in GPUs only because AMD lets you buy practically an A10-5800K with the IGP disabled. That does mean that the 760K does perform worse than the A10-7850K when both are benchmarked together with the same parts (besides the CPU, obviously).
So in short, you can go with either one. The 265 does perform better than the 750 Ti, but only by 5-10 fps more depending on what game you play. And the cheaper 760K lets you throw in a GPU under the $700 budget, but doesn't let you upgrade to a more powerful CPU (like the i5-4690 or i7-4790) in the future.
And the reason why I chose a single 8GB stick was to let you add in another one in the future, instead of having to buy another pair to put into the motherboard. Obviously the case needs some more fans if you like fans, but you can just get them in the future (I highly recommend Noctua, especially the Redux versions.)