If your processor is marginal, your gfx card will not run at full potential. But your CPU is fine at stock for the 280X. Increasing the CPU clock will improve gaming a bit in intense areas, but it isn't really necessary. Fun, but not necessary.
The non-K will OC via the base clock. But you will be limited as to how much. And everything else on the base clock will be OC'd too. So you'll have to back off on their clock multipliers. If you want to OC, go K. If not, save some $ and go with the non-K. At that class of processor, it isn't really needed.