What power suppy should I buy?

I have no clue what your build is, but I am going to say 500w should do you just well. the Rs500i, CX500M, SuperNOVA G2, or a SeaSonic should work just fine and allow you to overclock. I have a CX650M 80+ Bronze with an 6600k, and it's overclocked to 4.4Ghz
 
Best bang for buck is here.
PCPartPicker part list / Price breakdown by merchant

Power Supply: SeaSonic G 550W 80+ Gold Certified Semi-Modular ATX Power Supply ($59.99 @ Newegg)
Total: $59.99
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2016-12-27 14:33 EST-0500

If fully modular is a big deal then this is the next best but I would probably prefer the Seasonic anyway tbh.

PCPartPicker part list / Price breakdown by merchant

Power Supply: EVGA SuperNOVA G2 550W 80+ Gold Certified Fully-Modular ATX Power Supply ($69.99 @ NCIX US)
Total: $69.99
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2016-12-27 14:33 EST-0500