(short answer is at the bottom) it depends. do you have a GPU or are you just using the IGP? if so, spend the extra money on a Z77 mobo because you will need a BIOS update for your motherboard. cant do that without a CPU
if you have a graphics card, then get the 2500k. there isnt much of a CPU upgrade in IB, they focused more on the IGP which is now awesome, you can actually somewhat play high end games on minimum settings. you can actually get 30FPS on BF3. also, Intel cheaped out on the thermal paste they used for the CPU, so IB chips have insane heat issues when overclocked. you can only get to about 4.2GHz and keep low temps on air. even on watercooling, 4.5GHz still gets pretty hot.
if you have a GPU, get the 2500k.