Low Power NAS-like Device

Hey, lazyweb.

Recently I've been thinking about turning my desktop PC off to save electricity. Lately I haven't even been using it all that much, instead opting to sit in the living room using my Powerbook. However, my desktop PC currently hosts a public SVN, as well as some private web resources that friends use relatively often. I could move the SVN elsewhere (probably mocha.xfce.org), but the private resources are not feasible to move to an external host. I also use the box as a backup mail server, but I haven't needed it in some time (right now it's more of a backup to a backup).

My main issue is storage. I currently have about a TB of storage in my desktop PC, which acts as a media source for my HTPC in the living room. For various reasons (mostly physical space), moving the storage to the HTPC isn't really workable.

So, my next thought would be to build a NAS. In the process, I'd like to upgrade my storage to SATA, and use RAID5. However, this isn't strictly necessary; I'd be OK just moving my current hard drive setup to a new box and putting off the upgrade until later (which is more likely, as I don't really want to spend the money on 4 new 320 or 400GB hard drives right now).

So, here's what I want. A low-power NAS box that can take a minimum of four hard drives. It would be nice if could be expanded later to support eSATA so I could add external boxes if needed. The software running it needs to be completely open source (Linux preferred, though one of the BSDs would do), or easily replaceable with an open source NAS stack. I need to be able to run a web server on it, and it needs to be able to handle RAID5. Gigabit ethernet is a must, even though my home network doesn't support it (yet).

I also wouldn't mind building this. A low-power, heavily integrated motherboard with a low-power CPU and 4-8 SATA ports would be fine. One or two PCI slots would be good. If the motherboard doesn't have IDE channels, I'd need to get an IDE card for it until my storage gets upgraded. Ideally I'd like to be able to hook up a 512MB or 1GB flash card to it to host the OS so I don't have to waste HD space on that stuff.

Oh, and I don't want to spend any more than $350 or so on this (not counting hard drives). That might be tough.

Any thoughts?