I have an old dell server, might be a 600 sc, I don't remember ... bought it eaons ago during one of the hot deals (got the server with a pair of 2 x 120GB drives for about the same price as 2x 120GB drives)
It came with a 1.8ghz celeron, but at some point I stuck in a cheap p4 2.4 ghz (cheap as in $4 or something)
I only have 384mb of ram in mine ...
I used it for ip masquerade, ventrilo server, file server, ftp server, web server, database server, as well as some older dedicated game servers over the last decade or so.... Never got much volume since it was just a home server.... and it handled everything just fine.
You could use something a LOT less powerful than a 600sc for a file server....
Note: I've always run Slackware on the box, but Ubuntu server is pretty streamlined as well, so it should work just fine IMO.
If you have 256mb of ram or more, than that 600sc should be fine....