I meant auxillary power as in another molex. And I only plan on getting as low as about 30-40% fan speed, so the motors shouldn't have a problem.
I didn't realize it was a as common of a question as it is. But also as complex. Being fairly new to electronics (well creating it), I didn't realize it to be as complicated. I just assumed it would be simply connecting the PWM pin to the Arduino's, and supply them both with power through the PSU and witha change of a variable, that would be it.
Well time to research because I don't understand half the things they are trying to say.