My first thought was a 6670 just because it has low power requirements, but then I realized that a lot of pre-builds come with sub 300W supplies.
Here's what I'm thinking, should fit into your case:http://www.newegg.com/Product/Product.aspx?Item=N82E16814102958
But we really could use the rest of your system specs. That graphics card will be mostly wasted on an old athlon single core, and might not even run if you have a 200-250W PSU. Maybe a 250W, it gets shaky.
Another, cheaper option that might better fit your CPU is the GT430. This one includes a small form bracket, and will consume much less power, since we're in the dark on that so far. You also lose a lot of the graphical ability of the 6670.http://www.newegg.com/Product/Product.aspx?Item=N82E16814187121