The 2500k is more a good deal faster than any current AMD quad core but right now it might not really be worth the upgrade, the Phenom II X4s still perform well in games and can deliver over 60FPS when paired with the right GPU, generally the intel CPUs are more useful if you need the extra CPU power to prevent a bottleneck with the more powerful SLI or Crossfire setups, and for a variety of non gaming applications. You can upgrade if you want, but unless you are facing a severe CPU bottleneck with your Phenom II you probably won't see a noticeable performance increase in games, and you do have the option of overclocking your current CPU if you do have a bottleneck.
As far as gaming performance goes there is virtually no difference between the 2500k and 2600k, the 2600k features a slightly larger L3 cache, a slightly higher stock clockspeed (100MHz faster) and hyperthreading. Hyperthreading and the extra cache don't help give any better performance with games, and the extra 100MHz can easily be matched with a very slight overclock on the 2500k.
Faster cores will obviously lead to better performance and can help alleviate bottlenecks with the higher end SLI or Crossfire setups, but an extra 100MHz isn't going to make a huge difference, particularly with a CPU like the 2500k, which can destroy any game you throw at it.