Seriously, I can not see any problem with upgrading to SP3. You could just image with Norton ghost the computer and then apply the upgrade. In the future you may have to go with SP3 to insure software compatibility just like a lot of current software requires WinXP SP2. SP3 gives the computer a features freeze in that you would know that every required update is actually installed whereas just having regularly updated the computer does not give you that assurance.
There are some additional networking and permissions features that are rolled out in SP3. I believe these features exist to allow better integration with Server 2008. I am not sure what the significance of these new features are.
This wiki explains some of the features:
http://en.wikipedia.org/wiki/Windows_XPI am not sure that you are required to do so but (knock on wood) service pack upgrades are not too much of a problem in XP. Vista SP1 trashed my computer, requiring a reinstall. You will need to upgrade all of your computer if you decide on SP3, a mixed environment is a bad idea.