I am looking to upgrade my computer's video card and ran into a problem help please?

My current computer is a Hp S3500F and the only bad thing on it is the video card so i am looking to upgrade it with one, I then discovered that you would have to buy a special kind of video card and a new PSU, I then decided that i was just going to buy a new case and put the insides of the S3500F into the new case and upgrade the video card/ PSU from there. I have around $250 to spend on the computer overall and i need to know what would be the best setup for my computer and what would be compatible. I would like to spend about 150-170 on the video card max. And preferably a 300 watt + PSU.|||i dont know if this will help because these guy are verry expensive go to alienwear.com they have the best video cards around