Remote Linux Backup???

- 1 x Remote Linux Server in Internet Datacentre with lots of hard space (miles away from my office)

- 1 x Windows 2000 Server (in my head office)
- 20 x Windows XP/2000 Client Machines (in my head office)
- Head Office connects to the Internet via a 1MB DSL line
- Head Office has staff of 23

- 1 x Windows 2000 Server (in my branch office)
- 5 x Windows XP/2000 Client Machines (in my branch office)
- Branch Office connects to the Internet via a 512KB DSL line
- Head Office has staff of 5

What is the solution for providing
(a) remote backup capabilties such that DATA (not OS) from the 2 servers can be automatically backup to the Linux Server in the datacenter via a schedule - this should provide some form or ability for keeping different versions i.e backups from various days
(b) remote backup capabilities for the 25 clients such that they are able to select the different folders on their machines for backup and be able to control some form of schedule
(c) GUI client interface for the windows server and client machines

Any help?