Kevin Kelly's Cool Tools recently had a write up at http://www.kk.org/cooltools/archive/003428.php
regarding Generac Guardian Automatic Standby Generator. It is more detailed than I could be on the subject. The most import thing is making sure that the generator output (kW/hr) is greater than your probable consumption during a blackout.
When determining which systems will run off the generator, consider other creature comforts, such as running the hot-water heater and some/all of the electrical outlets in the kitchen (for charging cell phones, making coffee, etc.); however, such additions will add to consumption calculation.