If you don't run applications that use CUDA or OPENCL or run GPU stability testing (e.g. FurMark) then you may be able to get away with a 650W PSU depending upon which brand and model of PSU you have. The PSU will also need to have a sufficient number and type of (i.e. 75 Watt 6-pin or 150 Watt 8-pin) PCI Express supplementary power connectors.
A 750W PSU should definitely be enough.