I would have to agree with Chef. SGI doesn't use so many fans, and with their price tag they wouldn't mind adding a hundred fans if it improved their performance. Only when you get into massive cooling techniques like
www.Kryotech.com will you notice it with their thermal acceleration.
OC'ing doesn't gain any noticible increase, even in the popular Athlon 600 to 1000 OC. Clocking utilities still find it pretty close. You will probably spend more on cooling then just getting the better processor.