Upgrading to sp3 should have no effect on WGA
however before you start make a copy of the activation files
Go to the C:\Windows\System32 folder. (You may have to click on the link that says "Show The contents of this folder.")
Find the files "wpa.dbl" and "wpa.bak" and copy them to a safe location. You can copy them on a floppy drive or burn it onto a CD or DVD.
After you have updated Find the file "wpa.dbl" and "wpa.bak" (if it exists) and rename them to "wpadbl.new" and "wpabak.new."
Copy your original "wpa.dbl" and "wpa.bak" files from your floppy disk, CD or DVD into the C:\Windows\System32 folder.
Restart your system. (If you followed the directions in Starting Windows XP in SafeMode, you may need to go back into MSCONFIG to turn off booting into SafeMode).