For a system using a single GeForce GTX 630 graphics card NVIDIA specifies a minimum of a 300 Watt or greater system power supply that has a maximum combined +12 Volt continuous current rating of 20 Amps or greater.
Total Power Supply Wattage is NOT the crucial factor in power supply selection!!! Total Combined Continuous Power/Current Available on the +12V Rail(s) @ 50°C ambient temperature, is the most important factor.
Overclocking of the CPU and/or GPU(s) will require an additional ??? Amp increase to the combined +12 Volt continuous current ratings, recommended above, to meet the increase in power required for the overclock. The additional amount required will depend on the magnitude of the overclock you are trying to achieve.
The GeForce GT 630 draws less than 68 Watts even when running a GPU stress testing utility like Furmark. The 300 Watt figure stated is for the system including the graphics card, CPU, motherboard, hard disk drive, optical disk drive, CPU cooling fan, case cooling fan, etc.
That OCZ GameXStream 850W (OCZ850GXS) is a discontinued product (i.e. EOL - End of Life). In fact the whole GameXStream Series was discontinued by OCZ. They had a very poor reliability record and the PSU cooling fan was considered loud.