For a headless box, I would suggest Debian, Slackware, Ubuntu server LTS, or Centos.
Your hardware is defiantly massive overkill
I like it
My server is a dual xeon with 2gb of ram, but the only reason it is so high spec is because I won it on ebay for 500 bux about two years ago
With the specs you are planning on building, you can have fun running virutal servers on top of your real one if you like.
Whatever Distro you choose, make sure to set up automated upgrades as well as the automated backups you spoke of.
You may wish to consider software raid. It is very easy to set up, (especially with ubuntu's alternate install disk) and you can do some very cool things that cant be done with hardware raid with it. For example, It is possible to take a pair of software raided drives and move them to a newer, faster server without a reinstall. It is also possible to intentionally "break the mirror" for the pourpose of testing out major software chages, and booting off whichever side you decide to keep to resync the box.
As far as the extra fan, It might help, it might make no difference at all. It cant hurt unless you put it in in a really stupid way