only z68 has cpu overclocking so it goes with the "k" version of i5 2500k. its more expansive by at least $50.
other i3 and i5 does well with h67 since its cheaper then z68 with same functionality. H61 is cheaper still but its missing some function. Really depend what you need it for, so if you can tell us what you need it for, people can better help.
zhihao50, like I said, I am looking to build an office rig, with emphasis on processing power rather than graphics.
By the way, am I right in assuming that the on-board IGPUs that come with certain i3s and the i5s are capable of offering smooth playback of 1080 HD videos, because, at this point, as far as graphics processing concerned, that is my only requirement. I may install a graphics card in the future, but that is not a priority.
if your budget have at least $350 for motherboard and CPU then
i5 2500k ($220) plus asrock z68 + extrene gen 3 $125 would be the way to go.
it got hd3000 which is a stronger igpu, powerful CPU and ability to overclock later on.