Did someone say FreeNAS?!?!!?
The DIY route is always an option. With something like freeNAS (or even a regular linux distribution) you can use an old PC and turn it into a NAS box that you have a lot of control over. If you use gigabit ethernet things really seemt o move pretty fast. I can stream at least 2 1080p videos from my box at the same time (never tried more than 2

).
The upside is the total control you get. You can use SMB (windows sharing), ssh (secure) sftp (secure), apple talk (mac), or any combination of the list to share files. You also tend to have more control over user access, etc.
The downside is that you have total control. My NAS servers have been (knock on wood) reliable, but if your using older hardware, you could have a failure. The other downside is power, an older PC will take more power (and put off more heat) than a simple NAS box.
In terms of external enclosures - I'm loving eSATA for both desktops and NAS servers- I like the Icydock stuff. Newegg.com seems to have the lowest prices, although they can be found on amazon too.