Welcome to the forum! We like noob questions! Only the first two G5 iMacs ran 10.3 so check that your Machine ID is 8,1 (About this Mac, More info, Machine ID) As foidulus says, FTP doesn't require OS X Server, but if you have a license sitting around collecting dust, then go for it! Here is a list of FTP servers and clients that will run on a regular (non-server) OS X system:
http://www.pure-mac.com/ftp.html