The 2500K also has better integrated graphics (3000) over the 2500 (2000), but this is of little consequence for those using GPUs for gaming, of course...
At medium resolutions (1280x1024, 14x9, 16x10etc..), the 2500/2500k will provide higher framerates, sometimes up to 50% higher; once the resolution is up to 1920x1080/1200, the framerates are much closer as the GPU normally becomes the limiting factor. On a pure gaming CPU 'horsepower'/throughput contest, the X4 OC'd to perhaps 4 Ghz will then keep up with the 2300 at 2.8 GHz....maybe.
1. "K" has unlock multiplier for better overclocking
2. "K" has integrated Intel HD 3000 video core instead of the slower HD 2000 video core.
3. The non "K" version has "TXT" and "VT-d" functions which basically make it more difficult for a hacker to gain access to your PC and steal certain types of data from RAM. This is meant more for businesses rather than home users.
The i5-2500 (either version) is more powerful than the Phenom II x4 955 in basically all categories. Even the dual core i3-2100 will give you better performance than the Phenom II x4 955 in games.
See following comparison between the i5-2500k and the Phenom II x4 955; the i5 wins every single benchmark. Note that in some cases lower score are better than higher scores (such as rendering time), while in other cases higher score are better than lower scores (like frame per second).