Speed Controlling a AC blower fan?

I have a duct fan that usages draws 2 amps. I plan on adding it to my HVAC system upstairs to draw the cooler air downstairs up during the hot summer. I have the ethernet shield and all that good stuff. I read about relays, triacs and what not but not sure where to start or which is the best approach. Can someone recommend a good starting point?

The speed of an AC motor is controlled by the AC line frequency, which you can change using a Variable Frequency Drive. You can also buy fan motors with two or more windings, which give a limited selection of speeds.

If you want full control of the fan speed, and don't want to pay for a VFD, it might be cheaper to replace the motor with a DC treadmill motor and use a simple DC motor controller.