If you're not going to buy the 6600k, I'd say buy the 6500 and save some one the motherboard side too, by buying a cheaper chipset like h170 or b150 instead of buying the 6400. The 6500 is a sweetspot for budget minded rigs, as it doesn't cost that much more than the 6400 but has more performance that makes it definitely worth it. The oc won't even matter for the cpu side if you're planning on mostly gaming, and the graphics card should be the main stuff you're spending on.