300W is pushing it and even then it's probably not the quality of a corsair or XFX, I would say you would be much better off buying a new PSU and it will last you much longer on the long run.
New PSU will open you up to more powerful video cards, but honestly, anything over the HD 7750 will be bottlenecked by your CPU.
I used an overclocked HD 5670 in my Dell with the stock PSU (same as yours) for a year and it was fine - silent even. The HD 7750 draws less power than the HD 5670. I'd be surprised if your system + the HD 7750 draws more than 125w at load. And Dell uses good PSUs.