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.