I built my own Windows Home Server box. You can buy some decent pre-built units, but I wanted to build my own. I wanted a small, quiet, low-power device. I was willing to compromise on performance & capacity to get it. I decided that my collection of movies should live on my media center, just because I wanted to have a small WHS.
Dell D600 laptop
A laptop meets the small & low-power requirements without effort. They also have a built-in keyboard/mouse/display and battery backup, while still being small.
Dell leases these to corporations. After the lease expires, they are sold for cheap. I bought mine at a popular auction site for ~$300. It had a 1.4GHz P4 and 512MB RAM. Most have USB 2.0 (important for adding storage) and GigE (important for pushing a lot of data back and forth). Many have Wi-Fi, which may be useful.
D600 parts are easy to come by, and their online manuals are very good.
Pair of 160GB 2.5" drives
Hard drive prices (per GB) follow a "saddle" curve. At the time of building, 120GB were at the bottom of the curve; 160GB were a bit higher. Above 160GB were much higher. I knew that I wanted space for 100GB today; 160GB would give me a little breathing room. Since replacing drives is expensive (and replacing the primary drive is annoying), breathing room seemed wise.
For the secondary drive, I bought a 2nd HD caddy, which replaces the DVD drive.
Great print server!
It's small enough that it can fit comforably under my printer on my desk. (I was careful to leave a little venting room.) I like simple, obvious names, so my printer path is now \\SERVER\Printer. The USB cable from server to printer only spans ~6 inches.
What if I need more storage?
One of these days I will probably decide that my movies should be moved to the server. I rip my DVDs lossless, and currently have ~400GB, so I'm going to need more room. Upgrading the internal 2.5" drives won't be feasible for a while - they're still too small. Until then, I will need to add external storage.
I can use the pair of USB 2.0 ports. With these, an external enclosure with a pair of large 3.5" drives are a good choice. (Two drives means I can fully use the USB bandwidth without dramatically overloading it.).
Another option is to get a PCMCIA card (there's only one slot) with USB 2.0 or eSATA.
One downside is that the external drives won't be on the battery that's built in. That means I will need a separate UPS if I want them protected.