I'm waiting for OS X Lion Server to be released to install it on my home server (really just a mac Mini running 24/7 next to my router). Since most of my sharing (calendar, docs...) is between my wife and myself, it seems doable.
I'm just not sure that iCal, Mail and the file sharing features will be adequate enough to replace Gmail, Google Calendar and Docs.
However, it is a pain. I remember sitting in a hotel lobby in Saigon next to a guy who was trying to revive his ailing mail server in London while in the middle of his honeymoon. I myself am still serving web pages and forwarding email for a handful of friends and family. When things go wrong (despite many levels of redundancy) I have to sacrifice my valuable time for what could be somebody else's problem. It sucks.
Besides the Four Fundamental Freedoms of Free Software, people have an additional freedom: The freedom to give up their freedom. In my case, my life is greatly simplified by giving up a little freedom and having Google's sysadmins manage the hardware and software for a lot of my personal data. The risk is even less when places like Google and Facebook allow you to export your data.
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.
(I have no experience with it myself)