For overclocking you will definitely want to spend a little bit more and get the i5 2500k. It is extremely overclockable and, for the price, the best CPU on the market. For $20 more you gain a substantial amount of unlocked performance.
The 7970 is a great choice, it really just depends on, as you said, whether you want to spend the extra cash. The GPU is the most important part for gaming, so it would be justified if you decided to go that route.
After a quick glance, other than changing the CPU and picking either card your build looks pretty solid to me.