Even 32-bit XP does not deliver 4.0GB. You should see it deliver from 3.0 to 3.5GB since some of the 32-bit address space is used for the video and other spaces.
But let's move back to reality. Today your best machines are running on 2.0 GB. I've yet to find a real reason to go beyond this.
There is also that 2.0GB Application Memory Limit in 32-bit XP (research the /3GB boot.ini switch.)
hey i really need some help im building my first custom pc and this motherboard im looking at says maximum memory is 4GB- 32 bit. does this mean only 32 bit OSs will recognize up to 4GB??? What about 64 bit??