Depends on your needs really...
Gaming, Photoshop, or just basic web designing.
AMD graphic cards are generally less power consuming than Nvidia.
Cards under this category on that page will fill your needshttp://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=100007709%204026&IsNodeId=1&name=%2450%20-%20%2475
In my opinion you should upgrade your psu to atleast 500 watts.
You should go for Direct X 11 as its newer technology.
Nvidia cards 200 series and below are direct x 10
AMD cards 4000 series and below are direct x 10.