I will be using the system for gaming, video encoding, etc.
Games (so far): Borderlands, TF2, Skyrim 3? at 1920x1080.
Other programs: Handbrake, Audacity, Office stuff.
I was kind of leaning toward the 2500k with the thought of an upgrade to IB if/when needed. Just want a decent mobo to go with it that will OC well (not MSI). Or if the price is right with the $80 discount on the mobo, I would consider the 2600k.
Z68 is a good platform choice. For gaming and audio editing (audacity) the 2500k is the sweet spot and a great choice. Handbrake on the other hand will eat as many threads and ghz as you can throw at it. The 2500k will do well, but the 2600k will do considerably better ( and certainly not hurt the gaming).
So. If cutting perhaps significant time off handbrake encoding is worth $50 to you, go with the 2600k combo - otherwise the 2500k combo.