Budget and purpose?
Far as a cheap replacement, this would be the easiest since you just pop it in, and don't have to worry about drivers:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814133386
Far as the best replacement, The ATI HD6670 is the fastest card you can get without replacing the power supply:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814161397