You're going to need a new power supply for sure. But I don't feel like dumping $200 into that computer would be economical. Here is what I would do:
Get a new power supply and a nice video card, like the Corsair CX500 and a Radeon 6850. Then run them in your current computer for awhile until you have enough funds to upgrade the RAM and CPU in your computer.
Get a decent i5 quad available for the cheapest price....OH and if you're going to get the i5 or an i7 you will need a new mobo as well...because socket 775 isn't suitable....so atleast you can get an old quad core or core 2 duo...and I don't think you have to upgrade your RAM because 6 GB is enough...then get a good GFX card like HD 7850 or Nvidia GT550/GT 560Ti...then you need to get a good PSU...go for an Antec, Corsair, XFX or a good cooler master 500W+ with two +12V lines and two 6pin power connectors for the graphic card...Good luck