There really is no point in getting a 512mb card, most current card usually have a minimum of 1gb.
I'm also going to assume that you have a 250w power supply due to the nature of your computer.
Assuming you chose 512mb for a really cheap card, maybe something along the lines of a 6450.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814125404
If you plan on doing any sort of gaming, I would recommend a 7750. That would probably give you the best performance your power supply can handle.
Maybe something like this.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814131461