The core i7 has 2mb more L3 cache and 100Mhz quicker CPU power and has hyper-threading, so not much. In games, the 2500k and 2600k are basically the same CPU. Most games won't use the 8 threads the 2600k has to offer, so the 2500k. Also, if you have a discrete GPU, and don't plan to OC, the 2500 or 2600 is what you want. The k series has hd3000 iGP instead of the non-k series iGP with a hd2000 GPU and an unlocked multiplier needed for overclocking. So, basically, the 2500 and an SSD would yield better performance overall and if you have a discrete GPU and don't plan to OC, the 2500 would fit you fine over the 2500k.
Hope this helped!