Power Supply Internal 250W (100-240V) Form factor: internal ATX
Total Wattage: 250W
You need an ATX power supply. There are some really good ones and some really really bad ones and they cost about the same for some reason. Here is a good cheap one: Antec VP-450 - generally under $50. If you are buying a different one post over in the power supply forum. A bad power supply can give you un-debuggable intermittent software errors as the CPU and MB give the wrong answers back to SW.
Once you have the PSU installed use a reference like this one. http://www.tomshardware.com/reviews/gaming-graphics-card-review,3107.html
to pick the right video card.
At $300 minus $50 for the PSU you can get a monster of a video card which may have length issues and clearance issues. Paying less gets you a slower card that is shorter and does not game as well. In your price range look at HD7850. The best way to tell if it will fit is open the case and get out a tape measure. See what blocks the video card from running full length.