I'm running the Waves Ltd. Ultra Pitch plug-in currently on my home system. I'm running Pro Tools 9 on a Mac Book Pro computer that has 4 GB of RAM and a 2.2 Ghz Intel Core 2 Duo processor. It's definitely been enough for me to be able to run this plug-in without any issues on this system, using only minimal instances due to the nature of the plug-in. I think it would work well on any system where you're currently able to run other high quality plug-ins smoothly.
The Waves Ltd. Ultra Pitch is definitely one of the best tools out there for harmonizing and pitch shifting in the box. This is a tool for tuning vocals, really just for basic shifting. It's pretty simple to use and definitely has enough to keep you busy for a while. Between the six harmony bands, you should be able to layer on some pretty thick digital harmonies, or simply double up a lead vocal if need be. It's a nice plug-in to have but isn't one I would have bought as an individual. It's the kind of great plug-in that beefs up the value or any bundle as it's a great tool to have for anyone since you never know what is going to come up. If you have any Waves bundle, you probably have Ultra Pitch - check it out if you haven't already.