Hi, nemeisis1.
Depending on the booth size, your computer fans are probably going to have to run at max. rpm to provide the capture velocity necessary for vapor transport. Keep in mind that these fans are designed for cooling electronics components in a very small area. (My computer tower is 6" X 16" X 17", which works out to slightly less than 1 cubic foot, and there's minimal air movement at the computer face a mere 17 inches away.) Obviously, as a result, they don't need to move very large quantities of air.
Personally, I would opt for a larger, more efficient fan design; however, if you're dead set on using these fan types, you might want to consider a downdraft design rather than a back- or laterally-drafted booth.
Before opting for a rheostat (speed controller) installation, you need to know the face area of the booth you are constructing, and the total cfm necessary to move contaminants. If your opening is 2ft. X 2ft., the total face area is 4 square ft. Airflow at the face should be about 100fpm (feet per minute). Therefore, total cfm is 4 square feet X 100fpm = 400 cfm. Depending on duct length and design, filter size and thickness, etc., you may want to bump your cfm rating upwards anywhere from ~10-25% (roughly). This is for a back-drafted booth.
For a downdraft, you can cut your cfm requirements some due to the fact that you're using gravity to help remove vapors. But in the case of downdraft design, you should consider installing a plenum to even out the airflow throughout the base of the booth. See my discussion with Rich Carpenter at /forums/1185586/ShowPost.aspx to get an idea on his plenum design. But you're still going to need good airflow in order to evenly pressurize the plenum.
Good luck with your build, and keep us posted on your progress.
Gip