WSUS setup on SBS 2003 R2

I am in the process of setting up an SBS 2003 R2 server. I have never used WSUS before. I installed WSUS 3.0. Now I am just trying to figure out what updates I need to check for WSUS to manage.
Do I need WSUS to download Server side updates, or just the updates for the client machines? like XP and Office updates.
Thanks,
-Stevens