First--you will have to look and see exactly what PSU you currently have--before we know if your money is better off spent on the PSU or graphic card.
The dilemma in this price range is that most cards up to about $70 can run on a generic power supply--once you go above that you often need to upgrade the PSU. So--depending on your PSU quality it might be worth it to replace, or simply get the best card you can with the current one.
Also, wattage is not always the best measure of PSU strength-- many cheap PSU makers crank up the wattage numbers-- but make inferior models.
If you end up replacing the PSU and getting a graphic card I think the best deal you can get in the slightly over $100 price range would be a $39 corsar 400cx PSU and a $79 Nvidia 9800GT.