thanks for the quick reply, cdburner
Molex adapters don't have a tach terminal, so am hoping Mobo would give it on default voltage.
The 180mm fan seem to have a resistor on its low speed, but i would not put one on the 120mm. As constant Auto mode would seem more viable if i where to slow it down. As it would be like putting a resistor on a auto regulated fan. It would supply more voltage to get the same desired RPM.
3pins nowadays auto regulate fans through voltage supply thus not needing the 4th pin anymore. 3pins are without power loss and electrical noise disadvantages.http://www.tomshardware.com/forum/266476-29-difference-3pin-4pin-connectors
If the tach switched off would kill the fan might as well turn it on all the way or ditch the switch
if you are handy with electronics, you could have your switch switch in a resistor to slow the fan down
just a hobbyist