Radeon HD 7750 On 300 Watt

I also have a 300W power supply and i used this calc

http://extreme.outervision.com/psucalculatorlite.jsp

And a 300W psu should be able to handle a 7750 just fine as long as your not using a wack load of fans/lights and a bunch other stuff in your PCi slots using up power

The 7750 is very efficient and uses barely any energy anyway only 55W. I had a older video card that was much worse than the 7750 and used way more power but worked fine on my 300W psu.