I have a very small 1tb NAS box with duel gigbit lan using an msi axis. I built with a mobile core duo chip so its light on power and cost under £200 for everything bar the hdd's.
It runs on hardly any power (all drives spin down etc) and a very low power mobile chip so leaving it on 24/7 is no problem.
It runs ubuntu and is a wireless router with firewall it also has 2 media centers wired off the spare gigabit port for HD storage, it also:
*Runs azureus with webfront end for torrents (I recommend this)...I have torrentflux b4rt installed its better than plain torrentflux but it still sucks imo (no global speed limits etc).
*It runs Ninan which is a web based usenet downloader
*It runs squid for an encrypted proxy server I use from work
*It runs jinzora which is a web based music organisation thing....I basically can stream my music from home where ever I am in the world...as long as I have a net connection, I use this in work all the time.
*I also use webmin to keep track of the whole thing, and also extplorer which is an excellent webbased file manager.
It does a lot more to boot but I won't bore you anymore