Well Virtu Lucid technology is still in development, the fact that Z68 boards allows you to fully utilize the CPU's iGP is a bonus, one of which is video encoding. I usually turn it on for efficiency reasons.
P67 boards doesn't allow you to overclock the iGP on the CPU, since you don't even have access to it. If by dedicated you mean discrete GFX cards, then this has very little to do with the motherboard. Provided you have a capable GFX card with decent cooling and headroom then this is usually controlled by the software readily available by the card manufacturer or programs found on the interweb.