S5 is around 600W normally, so get yourself at least 750 or even higher psu. The higher the better since efficiency tends to be best between 60% and 75% of max output.
so 750w psu can run it wit OC right ?
Yes, Ive been using a mix of Corsair CX750M and Thermaltake & Rosewill 850W PSUs to run them, overclocks fine.
If youre looking for a cheap PSU, Ive got a bunch of the above models for sale. PM me.