Where are you from?
The GTX 980 Ti is going to be incredibly bottlenecked by an A8 6600K APU, but if you plan on upgrading the system one component at a time or switching to something more powerful in the future, I'd say go for it. Also, take into account that the GTX 980 Ti is about 20FPS faster than the GTX 980 - if you only play at 1080p (I doubt you play at 4K with an A8 6600K) you'd be better off buying a GTX 980 / R9 290X.
The integrated GPU inside the APU should disable itself automatically when you connect the dedicated GPU. Yes, some overclocking would be a must for that APU.