A couple additional notes:

1. Make sure the rear derailleur you buy is not a 10-speed-compatible DynaSys version, because those won't jive with your 9-speed shifters. The cable actuation is different with DynaSys than with previous generations.

2. Replacing your whole crankset with a new one of the closest equivalent model would be as cheap or cheaper than buying the three chainrings alone. Weird how that works. The FC-M430 square-taper: http://www.amazon.com/Shimano-Alivio.../dp/B003ZMH5JY