I just upgraded my new system to max the RAM at 4 GB.  I, too, can only see 3 GB displayed in the Windows XP Pro system properties.  I e-mailed my computer maker about this and this is the response I received:

 

You want to see physical ram displayed in system properties. That isn't how 

Windows works though. Windows displays virtual address space there instead. 

It only says "RAM" though and most people assume it refers to the physical 

amount of ram installed in the PC. This seems to be confusing for many 

people as you aren't the first person to make this mistake. 

The 3gig number you see in system properties is virtual address space. It's 

not referring to physical memory. 

"All processes (e.g. application executables) running under 32 bit Windows 

gets virtual memory addresses (a Virtual Address Space) going from 0 to 

4,294,967,295 (2*32-1 = 4 GB), no matter how much RAM is actually installed 

on the computer." 

There's a long explanation here: 


http://support.microsoft.com/default.aspx?scid=kb;en-us;555223 
It's not a simple explanation though but it is the official explanation. 

Just understand that 3 Gig isn't referring to physical memory but virtual 

addressing space. Windows still recognizes and uses your total physical ram. 

It just doesn't display it in system properties where you thought it would. 

 

So, our systems do use and recognize 4 GB but not in Windows system properites.  It does in the BIOS and the Windows System Information page (START, Assessories, System Tools, System Information).

 

Hope this helps,

 

Ed