you might want to check out the NSLU2 from linksys. its cheaper and the possibilities are just endless

http://www.nslu2-linux.org

i have 2 of them:
1st: 2 X 250gb HDD running unslung firmware
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itunes server (firefly) - serves mp3 to any pc connected to my network via itunes
upnp server (ushare) - serves movies/mp3/pics to my media center pc in the living room, SLA5520i philips wifi hifi in my room, digitial photo album that i made from an old laptop hanging in my living room and my friend's ps3 (his unit is in front of mine)
torrent downloader (enhanced-ctorrent)
ftp server (vsftpd)
ssh (openssh)

2nd: 1 X 500gb HDD running debian (2nd usb port with usb-ethernet adapter)
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transparent proxy to cache all my traffic (squid)
backup my data from 1st NSLU2 and from all computers on my network (rsync & rsnapshot)
webserver for my online photoalbum and online calendar (mysql & php via fastcgi)
ntp server (openntp)
rss downloader (tiny rss)

i will totally agree with others that using an old pc will be cheaper and more flexible. freenas is one of the popular choices for this, ebox platform is also good (http://ebox-platform.com/features). but if you want something that you can leave running forever without having to worry about electric bills soaring then a true NAS appliance is still the way to go. hope this helps, cheers!