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.
Woah! that was a really good reply, cleared a lot of things for me.
One reason I'm upgrading to the 2500k is because I think my AMD will bottleneck the 6950 gpu I'm getting and also because the Sandy Bridge E and Ivy Bridge would remain out of my budget for quite a while after release. Since, I'm practically getting a new pc, I thought I'd upgrade my cpu as well.
Offtopic: Where would hyperthreading and the extra cache be most used?
What games do you play/want to play? I have an X4 965 and know very well which of my games are CPU bottlenecked while running one or two GTX 460s. There are some games that a Phenom II will bottleneck, even on the most modest of GPU and with the best CPU overclock. (And as a Phenom II owner, this makes me sad )
And to your other question, Hyperthreading most often gets utilized in professional settings, or by power users doing professional tasks such as video editing/encoding, 3D rendering, etc. Games certainly can use extra L3 cache, but the way the 2600k is set up, there is actually a slightly less efficient allocation of cache per core, and as a result, the 2500k wins at stock in games, even with its (slightly) lower clock.
Of those, I think Battlefield 3 and Crysis 2 would cause the Phenom II to struggle. Metro is almost always GPU bound when it comes to performance, it's not a very CPU heavy game. Battlefield 3 might be an issue if it is like Bad Company 2. A 3.5 GHz Phenom II X4 will cause maybe a 10-20% performance loss on a GTX 570 maxing out Crysis 2 at 1280x1024, probably much less of an issue if you are running a higher resolution than 1280x1024, at which point you will be GPU bound again.
the 2600k has hyper threading whereas 2500k do not, this means you will get extremely better performance in multi-threaded applications and games that can use hyper threading.
i5 2500k is faster than any AMD chip whereas 2600k is the fastest processor on the planet, even faster than i7 995x (2500k too), in terms of gaming.
the 2500k and 2600k are best if you want to max out FSX. Also, if you want use a lot of multi-threaded application, the 2600k will give you immense boost over 2500k, these processors are only meant for overclocking to any extent which is only possible in p67 or z68 chipsets.
But, if you still plan to upgrade get the 2500k and a fast graphics card, once again I am telling you, that if you just want to play games like crysis, gta 4, then any cheap but relatively good processor will do the job.