You will want to run it in a VM to separate the service from the rest of your system, if possible.
It needs very little CPU but, should have about 2GB of RAM due to a big Java component, and because it periodically kicks off programs to clean things up every few minutes.
A bit of a pain to set up, but once set up it works quite well. It might have too many features - if you want something a lot simpler, look for "qmail toaster" or a good guide to setting up Postfix.
