what is your budget, how much can you spend and at what point will you cringe and back off from spending another 5 bucks?
you say 'stuck with a 2500K' the 2500 is an amazing CPU that i would challenge any standard or even PC gamer to max out with out trying to max it out. its more CPU than most people currently need. the z68's are not much more and in some cases less than the p67 but you have trade offs as in the cheap 68's have a single pcie and stuff like that. what video card will you be using, what is your current PSU, and how many hard drives will you have?
Anort3 suggested one from "You need P67, Z68, Z75 or Z77.".
Please read this review....ASRock extreme 6 or MSI Z77A-GD65. Both are great Z77 mobos; the latest and affordable. You can overclock your i5 2500K. It is a great CPU to stretch it to the limit. But, get an aftermarket cooler (noctua nh-d14, cooler master h612 or corsair h100).