Network Server Capabilities

[sudowork]

I'm considering building a server for my home network, and I was wondering what all of the capabilities or add-ons were. I know the list is probably near endless, but I'm looking for suggestions prior to building it.
Right now I have the following in mind:
NAS/File server/Backup server
Print server
Firewall
Router/Gateway (possibly)
Web server (local and remote)
Media streaming (and TV Recording)
Mail server


Also, I'm wondering what are some of the possible software/hardware options for the aforementioned features. One of my other questions is what's the best operating system to be running. Note: I have windows, linux, and mac computers on this network.

Edit: One more question: What HDD setup should I use? I was thinking a RAID5 array consisting of 4-5 x 1TB SATA harddrives for general storage (yes I have a lot of media), and then a RAID1 array of 2x32GB for backing up. I haven't done any specific research yet, but are there any motherboards capable of having 7 or more SATA ports?