For a 750 GTX, 300 watt is completely fine. Ryarwood99 does make a good point. Quality power supplies do matter. Can you list out your entire build? Make and models matter.
APU:a8-5500 quad core 3.2 ghz with 7560d integrated chip
ram:4 gb 3.6 usable
psu i dont actually know what model it is just know its a asus power supply
You'd probably be OK. Just know that prebuilt computers (Dell, HP, Asus etc) aren't supplied with quality units. That doesn't mean wattage. It means, does the PSU have a quality build? Does it supply clean power? And more technical stuff.
I don't think you would have a problem. I don't recommend parts like this. You can try if you would like. You could get a good 450 watt Antec PSU for about $30.