Well, the PCs in my MAME cabs are completely de-cased, and mounted on a board, making them easy to remove when needed. This, and the fact an arcade cab is much roomier than a PC case, means I don't see the need for any fans. There's just the original vents, in the back. Never had a problem with overheating. Cabs have CRTs, too, which obviously generate more heat than LCD displays. Fans? Nah! I really couldn't be doing with an annoying fan whir constantly, either.