In round numbers XP can only address 4GB.
Your video card has XXXMB of ram on the card so
that limits XP to 4GB minus the cards XXXMB of memory.
In addition other hardware also takes away additional amounts of memory
(usually a small when compared to the video card)
and the remaining memory (address space) is available for
Windows XP and your applications.
So that 4th GB of ram you installed or will install is mostly unused.
Vista Users with SP1 now report how much physical memory installed on your
computer:
See: Windows Vista SP1 includes reporting of Installed System Memory (RAM): http://support.microsoft.com/kb/946003
RAM, Virtual Memory, Pagefile and all that stuff:
Basic information about the Virtual Memory implementation
in 32 bit versions of Windows 2000, XP, 2003 Server etc. http://support.microsoft.com/kb/555223