Note: Get the 550Ti if possible. Much better performance.http://www.anandtech.com/bench/Product/613?vs=541
Those cards are essentially the bottom end of the last generation, so to go down even further would put you two generations behind and it likely wont even be an upgrade to what you already have.
The only card I would recommend below this would be the GT210, and that would be to serve as little more than a display output, no way is it adequate for gaming.
Also it seems that cards lower than the 550ti and 6670 (apart from the 210) aren't even available on Newegg.