On your computer you won't be able to use all 4GB of memory, I'll explain.
XP Home is a 32bit OS and only capable of addressing a TOTAL of 4GB of memory, but that includes system memory and any on your GPU (I'm assuming at least 512MB).
In order to actually use 4GB of RAM you have to put a 64bit OS.
Hate to steal his thread, but I have 4GB of RAM and an 896MB card, does this mean I am not fully utilizing my VGA?