Will a Zalman 9900 cool a AMD Athlon X4 640 3.0GHz Quad Core,

Yes, it will cool your amd cpu just fine. But do some measuring and be sure it will fit your case. You can get a coolermaster or thermaltake standard 120mm heatsink for around $30 at newegg, so if money is tight, I suggest you use one of those.

yes the 9900 will cool it fine.