My file server used to run abecx.net for the longest time, its a 450MHz amd k62 w/ 128mb of ram, a 10GB, 120GB, 60GB, 80GB, and 40GB drives in it ( I had to buy a ide raid card to support the 120GB and the extra 5th drive ). It also has a cd writer and a dvdrom in it that I use to rip movies.
You dont have to have the baddest machines to run servers, about the only thing you are buying with a REAL servers is perhap realiabilty and redudancy ( extra power suppiles etc... ). IMHO, its better, faster, cheaper to use just some x86 that you have lying around, slap a network card in it, some hard drives and you are good to go.
What would be your purposes with this 'server'?