"This Old House" installed one during this season for a customer...you could probably find the details on their website. You need to verify that your flow is adequate so you don't starve the pump or you'll ruin it.
Functionally, you are emulating a well system with a storage tank. The larger the tank you select, the fewer the on/off cycles you'll need, but it might have the same issues as with a well control...the pressure would cycle between the high and low settings.
A flow-controlled pump that ran continuously when it detected flow might give you more consistent pressure, but would likely cost more...it also might not last as long since it wouldn't be on that long if you were say brushing your teeth.