I recently built a system precisely for this, and I too wanted to go as energy-efficient as possible. I ordered one of these:
http://www.newegg.com/Product/Product.aspx?Item=N82E16813181019. It's fanless, super low power, and has gigabit LAN, which you may want to have in the future.
I also ordered an IDE-to-compact-flash adapter with a 2 gig compact flash drive, and used it as my system drive. The OS is basically a SLAX liveCD server distro, except it boots off of the compact flash drive, not a CD (there is no CD drive in my server). This way the entire system drive is loaded into RAM at boot time, and no writes are made to the flash drive in normal operation - writes are only made if I change the server's config (a drawback of this is that changing the server's config will require a reboot). This is necessary to avoid wearing out your flash drive. I did have to manually setup a samba server on it (to enable filesharing to windows systems), which took a bit of work. If you're interested in setting up a system like this and have reasonable unix skills, private message me, or consult your local unix geek / unix forum for help.
Lastly, my server has two SATA drives - one for storage, the other for a backup. It's not a mirrored RAID 1 array. Right now it's just a periodic copy of what's on the storage drive, but I later plan to upgrade it to use an incremental backup system (rdiff-backup). That way even if I accidentally delete some files on the storage drive, or I want to go back to an earlier copy, I'm still covered by my backup's insurance policy . It also avoids super-long backup times, as only the changes need to be copied to the backup drive.