What is the model of your Dell computer? Dimension or Inspiron? I'm pretty sure your computer can handle a ATI HD 4670 videocard without the need for you to spend another $50 Dollars on a new power supply, unless you want to get a more powerful videocard.
Also, make sure that the videocard you get is single bracket and not double bracket (unless you like modifying the case to fit a double bracket).
You said your current Dell PSU is a little over 300W. So I'm guessing it's just like my Dell 305W PSU with a total of 22amps on the combined +12V rails.