Just a quick comment in that from an eletrical safety point of view, and what we call floating earths, you could cause damage to the PC by powering the fans from a seperate mains source as the fans could be a different potential difference to that of the PC.
This is a difficult subject to understand and the recommendation is that the PC and all its peripherals should be powered from a single source, a powerstrip, probably, in most cases. Fans which are an integral component are powered by the PSU and therefore will not have the forementioned issue. USB powered devices are the same as they too are powered from the PSU.
The integral PSU is common therefor too all equipment in the PC and all the equipment is at the same potential difference and no problem of floating earths can arrise. As would be a possibilty if you introduced fans, fitted to the PC case, driven by a different power supply to that of the PC's PSU.
Sorry for the long winded response but maybe you get from it that caution is needed. I.E. it is not a good idea.
