Inspiron 530s Graphics Card Upgrade?

I have owned a Dell Inspiron 530s for 4 years and finally decided to upgrade it. I've been researching for countless hours and haven't found anything useful about which video cards are compatible. The tower is nearly 4 inches wide (standing upright) and there's about 8 (by 3.5) inches of usable space within. Can anyone help me find something that's decently priced that will fit?