We are planning to set up a branch server (windows 2008) which will be linked to the main active directory server (running windows 2003). What bandwidth do I need to keep the domain synchronized? And is there anything else I need to take care of?
Update: There will be about 10-15 users using the domain.
It doesn't take much bandwidth at all to do this. Even site to site VPN over DSL connections would be fine so long as you're not making massive changes to AD all the time, which most people don't. The only thing that would take awhile would be the initial seeding of the AD database, and you can even use a backup from an existing database to seed it upon promotion if you're concerned about eating up bandwidth.
Another way to keep bandwidth utilization low is use Windows 2008 domain controllers in your entire domain, raise the domain functional level to 2008, and transition to DFS-R replication, which is significantly more efficient than FRS. But honestly, unless you're using dial up modems to link the two sites, you shouldn't have a problem.
Have a look here for a tool you can use to estimate your needs based on your AD setup:
http://www.petri.co.il/active_directory_sizer_tool.htm
With all said above, do keep in mind that you have to CONTROl replication time and frequency. It will work reasonably on anyhing above 64K Link.