Z68 = P67 + IGP or H67 + OC
And SSD Caching.
Anything else that makes it worth, because its kinda useless for someone that is getting a GPU??
You won't be able to use Intel Quick sync with P67 and H67 won't let you OC even if you have a "K" series cpu - Z68 brings both to you ie OC + Intel Quick Sync
Intel Quick sync offers the best video quality and speed in video transcoding :
The Sandy Bridge Review: Intel Core i7-2600K, i5-2500K and Core i3-2100 Tested - AnandTech :: Your Source for Hardware Analysis and News
The Sandy Bridge Review: Intel Core i7-2600K, i5-2500K and Core i3-2100 Tested - AnandTech :: Your Source for Hardware Analysis and News
So Z68 is really going to be just great to harness every features offered by K series SB cpus.