Actually it is O/S issue http://www.microsoft.com/whdc/system/platform/server/PAE/PAEdrv.mspx
Shows that XP is limited to 4GB whereas Datacentre is limited to 128GB! Once you start dealing with kit in this range ($80K+) you want to have all your RAM available - (why do you want 16GB? well if you have a 20GB database and it gets hammered you want in RAM)
http://support.microsoft.com/kb/283037 describes what happens with /3gb and /pae
If you really want to see problems - check out “Desktop Heap” http://support.microsoft.com/kb/126962 http://support.microsoft.com/default.aspx?scid=kb;EN-US;184802
The desktop heap is set on 32-Bit systems to 48MB! That’s all, and on servers where processes can run as users, it’s a problem. The limit is about 90 users (each starting out with 512KB used) on a box as more than this (48/.5=96) starts to hit the usage by system/service accounts. see http://support.microsoft.com/kb/169321 on how windows can start a new process in a new windows desktop