For a system using a single Radeon HD 7750 graphics card AMD specifies a minimum of a 400 Watt or greater power supply. The power supply should also have a combined +12 Volt continuous current rating of 19 Amps or greater.
Total Power Supply Wattage is NOT the crucial factor in power supply selection!!! Total Continuous Amperage Available on the +12V Rail(s) is the most important factor.
So your system have total 320W but it can deliver it all on the 12 V rail. So assume 290/12=24 Amps.
So to end it 50/12=4.1 Amp to run that card.
You are safe