Your computer supposedly comes with a 2ghz dual core and 1gb of ram. The processor is pretty weak by today's standards so it would be pointless to pair it with a high end card. 1gb of ram is not sufficient for current games, especially on Windows 7 so if you haven't upgraded to at least 2gb you'll need to and preferably 4gb if you can. The 300w power supply will be a limitation as well but it should handle most cards that make sense for your processor anyway.
So, upgrade the ram and for a card I would recommend something like an HD5670.http://www.newegg.com/Product/Product.aspx?Item=N82E16814102870