it's a 92mm temperature controlled fan with a proprietary dell connector to the MB.
have the exact same fan in my 400sc.
if you unhook the stock fan and replace it with one that plugs into a molex, you're gonna have issues at boot time with a 'cpu fan failed' message. you can ignore it, but you should know in advance.
the NMB is a pretty good fan, if a little loud....go over to the 400sc unoffical FAQ (google it) and you'll find tons of answers regarding dells. most of them use similar if not identical components.