Are you talking about a fan mounted on the cab itself, or a fan on the computer chassis inside. If it's the former, you don't need a fan on your cab. Monitors don't get hot enough to need one, and a computer has it's own fans, assuming it's still in it's case.
Which is another point. I really hate the advice that people take the parts out of their PC's case and mount them directly on the cab. WRONG! PC's are designed with the PC case in mind. The fans are placed in the case to promote proper airlfow. If you run a PC without the case, your certainly goint to shorten it's life as it'll get much hotter than normal. And adding a fan onto the cab itself won't help, as the inside is so large that airflow will disperse and won't be strong enough. The only reason arcades had bare mobos mounted on the cab was that they weren't very processor intensive, and the chips never got very hot.
If you must have a fan on your cab though, you might as well replace it with a quieter fan. SilenX makes some very quiet fans, as does Panaflo, Vantec, and Enermax. Search for "case fan shootout" or "case fan roundup", and you'll find loads of reviews on the net.