The subject behavior will occur if your computer has 4 gigabytes or more of RAM and you have the /PAE switch set in C:\Boot.ini.
Without the /PAE switch, a system with 4GB of RAM will show ~3.12GB in System Properties and MSInfo32.exe shows ~2,710MB of RAM, with and without SP2.
With the /PAE switch, before you applied SP2, these values were ~3.87GB and ~3,540MB.
After SP2, with the /PAE switch, these values are ~3.12GB and ~2,770MB.
This behavior occurs because of design changes to PAE mode that improves driver compatibility.