It could probably run without one, since the Athlon X2 4000+ is a 65nm processor and thus produces very little heat. I would probably put a low speed one in the back, just for peace of mind. Plus, it will help cool the hard drive and motherboard components through indirect airflow.
http://www.newegg.com/Product/Product.aspx?Item=N82E16835185056
These are nearly silent, and actually push a good bit of air for how slow they run. Since you don't have any especially hot components, it should do fine.