Building a gaming rig atm, and I've read extensive threads on these boards and elsewhere now, and I've learned that the only difference between i5 and i7 is hyper-threading which is no good for games. That being said would I be stupid to buy an i7 2600k for futureproofing purposes? does it make more sense to just get an i5 and OC it in the future? Im not concerned about budget but I don't want to waste money either, Im trying not to compromise on performance.
yes you are right. hyperthreading wont be of any benefit to games now, because they mostly use only 2-4 threads. Will they use more threads in the future? most certainly. If you look at intel`s plans, they already have prototypes of processors with something like 50+ cores. We might see stuff like that by the middle of this decade. More cores are the future of processing. I remember as recently as 2006 people were saying that you dont need to bother with those fancy dual core processors, because games only use one core. having said that, it may take a long time before games catch up with the hardware, and so whether spending the extra $100 for an i7 is worth it is all up to you. I personally think an i5 will last you a long time anyway, given how most games stress the gpu mostly.
The 2600k not only has hyperthreading, but, a larger L3 cache as well....and most 2600K processors have about 100 MHz more OC headroom than 2500k variants, with most topping out at 4.7 Ghz vice the 4.6 Ghz seen on 2500k when both are given the same high quality coolers on the same motherboard...
If forced to compromise due to cost, however, the 2500k is a very logical place to do so. (I'd much rather have the 2500k and a GTX580 than the 2600K and a GTX560Ti...)
Yes the main difference is that the i7 has Hyper Threading and a Larger Cache (actually it's the same chip but part of the cache and hyper threading are turned off on the i5). Hyper threading helps a CPU be more efficient. For example if one thread is busy doing a complex integer operation another thread on the same CPU can be doing a Floating Point operation assuming they are not using any of the same resources during that clock. It is true that right now it's hard for game programmers to make use of more than 3 or 4 threads, so the advantage of hyper threading for the i7 is almost non existent in current games but that will likely change in the future once the DX11 APIs threading capabilities are better and more widely used. The small boost by the L3 cache is hardly worth the price difference over the i5 2500k at the same clocks though.
Practically an overclocked 2600K will not increase playability of a game over a similarly overclocked 2500K in any setup. By the time an overclocked 2500K starts to seem lacking newer faster CPUs will be out. For a 2600K you might be postponing upgrading for an extra few months and you will have a status symbol for a while but that's about it. If you do alot of media encoding though you will appreciate the time saved by Hyper Threading, but if not then not so much.
If you do not have a budget constraint then surely get the i7 2600k. Its arguably the fastest desktop processor available currently. We recommend the i5 2500k in scenarios wherein the quite considerable price difference allows one to get a better video card which is a more important factor while gaming.