a lot of server tasks really dont need much power for just home use really
quite often, its the os itself that needs the bulk of the processing power.
im currently running a few servers for different things. One of them is a web/ftp and mail server.
it hosts a few websites, and several email domains, web mail, various spam filters and virus checkers, etc and a few other things. And it runs perfectly fine using a Via EN1200, which is a passive 1200mhz cpu. And 100w PSU in a 1u rackmount, althou it usually only runs at about 20w, althou not tested it in awhile
very rarely do i see the cpu above about 30% usage.
althou its running FreeBSD without a gui, and is mainly accessed via SSH