Those duct fans are useful only when helping a main fan do its job - If you have a long run of ducting or a couple of turns they can help keep up the velocity. Unfortunately the computer fan you posted will do even less unless you keep it completely unrestricted, eliminating any filtering you can do... The squirrel cage fans like mulletman posted are the best bang for the buck, although not the quietest. Anyplace by you do HVAC salvage for old heater fans?? Restaurant supply salvage, the fans from industrial range hoods are good too. I got a 1400 odd cfm blower like that (only its 240V) for free from a place by me

Cheap, powerful, and quiet are three words in the fan industry that don't add up - Pick two or shell out $$$