QUOTE(niai @ Feb 4 2010, 11:51 PM)
why would you want to do this?
QUOTE(FixItOrModIt @ Feb 4 2010, 09:06 PM)
I have it wired to a 12v source right now and it is insanely loud
That Rheostat would cost around $30~$40 or so and be huge. You can find them at Mouser, DigiKey and the like though, if you want to put a 25W heater in your PS3.
You could go with a 9v Regulator and a SPDT switch to go back and forth between 12v and 9v for the fan, or just pull the alternate voltage from somewhere else on the motherboard, make sure it can source that 2.5A first.
The advantage to using a switch is you know the fan will pretty much always be running at one voltage or the other, unless it dies or the wiring is bad. Something that has a knob on it that can be turned all the way down has a tendency to do so, even if you're not the one that does it someone sooner or later will, and that would turn the fan off unless you put a stop on it so it only turned so far.
Is there something wrong with the automatic fan control on your PS3? And I'd guess the reason there aren't a lot of PS3 fan mods are that's not really your typical fan, so replacing it with something that's easily available isn't really an option, though there are a couple of replacements out for it now.