I use a cheap pentum3 dedicated server in my basement. Also i would highly recommend http://www.bacula.org/en/ as soft-where to run. I do have a relatively good connection though 5mbs up 25mbs down. Works well because most isp give you a lot of downstream bandwidth. I only have 5 servers I am backing up though. Also using the bacula webmin module makes it stupid easy. Its also very secure cert and ASE security.
also make sure you a rsyning from you backup server aka: pulling from non backup servers. So if a security breach happens you don't lose the backups.