I would like to build a file server for my home network using Ubuntu.

It will serve files from RAID1 configured disks, either in the OS or in hardware. It will be connected to a Gigabit ethernet LAN. The disks will use an encrypted file system. It will serve samba shares.

I would like a recommendation on what kind of processing power/memory I would need to build a box that would be able to sustain the full capacity of the Gigabit ethernet connection in a file transfer for a single connection with the overhead of serving from an encrypted disk. I'm not looking to build a dream server, I just want enough processing capacity for high performance (and reliable) file sharing and spend as little as possible for it.

This may be tangential, but what kind of hardware would I need to have a server be able to reliably go into a low power mode when no requests are being made of it?