Mostly all 32bit OS's have a limitation on max amount of RAM. It is supposed to be 4GB completely, but the way MS designed the kernel you will never get that at all even with what people think is the reason the added Graphic RAM, which is not true as that works on a seperate channel.
On some occassions with specific motherboards I have seen 3.5GB max (even with a 256MB GPU so again both dont add up to 4GB).
On mostly all its usually around 3-3.2GB
On some specific 32bit Server OS's from MS you can get away with multi-channelled RAM to get 16GB.
But the norm to have 4GB+ you need a 64bit OS.
To show you mathimatically why...
32bit = 256*256*256*256 = 4096MB aka 4GB
64bit = 256*256*256*256*256*256*256*256 = You cant afford this so no point listing it :P