For a system using a single Radeon HD 7750 graphics card AMD specifies a minimum of a 400 Watt or greater system power supply. The power supply should also have 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!!! Sufficient Total Combined Continuous Power/Current Available on the +12V Rail(s) rated at 45°C - 50°C ambient temperature, is the most critical factor.
Overclocking of the CPU and/or GPU(s) will require an additional increase to the maximum 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 being attempted.
The Seasonic SS-400BT power supply unit, with its maximum combined +12 Volt continuous current rating of 30 Amps and with two 6-pin PCI Express supplementary power connectors, is more than sufficient to power your system configuration with a single Radeon HD 7750 graphics card.