[Al Skierkiewicz]

Brenn,
A few things came to mind when I saw your post. The output of any of these fan/generators will be pretty low as far as current is concerned but in order for you to use it effectively, you need to follow up the output with some form of voltage regulator. As the speed of the car increases so will the output voltage and current but the RPM of the fans is also determined by the electrical load. The greater the current demand the more electrical "drag" there will be. You might also want to control the amount of air flow as there will be a point where the fans may self destruct due to excessive speed. This can be a way to get charge current back to the battery but you will need some form of isolation to prevent the battery and alternator from back feeding the fans. The voltage regulator, if properly designed can handle this function.
Experimentation is key here. I would start by making a box with some lamps that you can switch in and out. Connect that to one fan and hold it out the window while someone else drives the car at specified speeds. Measure the current generated while you switch in additional lamps as a an increasing load and I think the data will be very revealing.
BTW, there are DC fans and there are brushless DC fans. The brushless fans will not act as generators.

Good Luck