When buying they should have dimensions. I will warn you though, on some feedbacks of these coolers (almost any) they wil say that it willblock one or ore RAM slots... If you got almost the whole computer from a garage sale, then the motherboard will probably stil have the stock cooler, and if it was a motherboard combo from someplace , usually it says that it is supposed to be replaced if you are doing anythign to put stress on the mobo (*cough*overclocking) or continuous use.
open up the side panel and blow a big ass fan toward it.
Or put a window ac next to it.. =)