This is purely from a Andy George perspective, and as I'm nowhere NEAR a linux guru, others might have a better view than I, but the following is currently in service for me at my feet, and working well...
2. Celeron 1.3 Ghz 256 Mb Ram 40 Gig HD
Your Celeron computer will do splendidly. Ram is always always a good thing though...
Disto is personal preference, SuSE or RH/FC works well for me, but the home server (Goliath) runs RH9. IPTables for a router. Configurable "Drop and Run" router scripts can be had from http://easyfwgen.morizot.net/gen/ and free...
Samba for a file server, thats a pretty straight forward affair, email me for an example of a simple/effective smb.conf file.
QMail is domain mail, works unbelievably well in this circumstance, and web interface for administration. Easy enough to set up. Guide for a QMail server from go to whoa... Have a read, see what you think... http://www.askdavis.com/qmailtoaster/
