It is not as future proof as you think. Sure hyper threading is good for certain applications that can use them, but real cores are always better.
By the time most games use more than 4 cores (of course some already do), your I5 2500/or i7 2600K will be an outdated piece of technology, as well as your other components.
Plus, with the 2500/2550K you can always OC the CPU easily up to 4Ghz with proper cooling, possibly higher with a high end air cooler, or a water cooler if you plan on going more than 4.1 4.2 Ghz.