If you'd like to replace that brake you first need to determine what "type" of brake system is supposed to be on there. Most likely a "single pivot" caliper or "Vee" brake (linear pull) depending on how it was mounted. Either has to be compatible with the "pull" of the handle. On a bike of that quality level you would like to pick up used parts somewhere like a Co op who could also help you out rather than buying a new part.

Here is a couple of good links to give you some reading if you are serious about working on bikes.
http://www.parktool.com/blog/repair-help

http://www.sheldonbrown.com/