P55 boards aren't compatible with an i5 2500k
Only H61, H67, P67 and Z68 boards are
Since you're getting a "K" series chip that are fully unlocked for overclocking, you'll want to get either from the P67 or Z68 chipsets to take advantage of it
z68 doesnt really cost more, but offers more. to be fair, it offers a lot that i dont use, but i would rather have it and not need it than need it and not have it. who knows maybe lucidlogix will get its act together and virtualized gpus will be awesome (im not holding my breath)
oh, in the very least... its nice to be able to use the 2500k gpu, just incase your main gaming card dies. instant repalcement
For strict Gaming there's no real upside to Z68, and while I appreciate Intel's RST (SSD Caching) your are limited to a 64GB SSD for the 'cache' and the costs are not too much more for a 120GB+ SSD which makes its' use pretty irrelevant. IF you produce a lot of MPEG-2/4/H.264 videos then sure get the Z68 -- otherwise P67.