one question, how come you have opted to go for a cpu with integrated graphics when you are getting a dedicated card. But to awnser your question it should be able to work fine with a 300Watt psu however you may want to upgrade to at least a 450Watt as the amd cpu has a tdp of 100Watts
The 7750 has the following power draw
System in IDLE = 155W
System Wattage with GPU in FULL Stress = 188W
Difference (GPU load) = 33W
Add average IDLE wattage ~10W
Subjective obtained GPU power consumption = ~ 43 Watts
So in theory your psu can power your pc, but make sure it a good quality one or just go out and buy a new 450 watt psu to ensure that your system is stable with these components