A UPS can be very beneficial, but like anything else in this arena, you should have a good idea of what you need, and don't go cheap. I use a CyberPower UPS in my rack with sine wave output, automatic voltage regulation, and a display that shows total system power output. I also have it connected to the network for remote management and configuration.
Some numbers to keep in mind:
1800W is the maximum for power delivery over a standard household 15A/110V circuit.
1440W is the recommended maximum for continuous power draw on said circuit. This allows for transient peaks without worry that the circuit breaker will trip.
When determining the power draw of your system, though, there's really no substitute for a meter. The power specs provided for the various pieces of gear will be maximum ratings and not necessarily what you'll actually experience.
My rack houses all of my theater gear, as well as network gear, two NAS's, and home-automation gear. The rack UPS also provides power to the theater projector (JVC). With everything on, and a loud movie playing, power consumption peaks at less than 775W. Of course, you need to take into account anything else on that circuit; but, the point being, it's less than most people think just by looking at all the gear in the rack, and less than you'd expect going by just the power ratings of all the gear.