Microsoft

There are a lot of updates in Windows 2003 SP2, which also shares the same service pack with Windows XP x64.

A few potential gotchas:

You'll want to uninstall IE 7 before upgrading from SP1 to SP2.
You'll want to install ADAM SP1 before upgrading to SP2 if you are running ADAM.
If you don't know what ADAM is, 99% of the time, you probably don't have it installed.

As a FYI, ADAM is getting renamed/reworked in Longhorn Server. The concept will still stay the same.