https://www.amazon.com/gp/product/B00F6EK9J2/ref=oh_aui_detailpage_o01_s00?ie=UTF8&psc=1
Check your electricity costs and run some calculations before investing in additional hardware, but it looks like you could get 3 graphics cards connected to that system. (You'd definitely want to take it out of the case and do some kind of custom frame.)
As above posters have mentioned, CPU mining would be for a learning experience more so than profit. For any serious profit, you'd need to consider what you'd pay for power and look into getting some graphics cards. (No guarantees, even then.)