Best card for <$100 would be a Geforce GTS 250 or a Radeon 4850. However, on your 300W power supply, the best you can probably use is a 9400GT.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814500124
http://www.newegg.com/Product/Product.aspx?Item=N82E16814500124
