i would honestly have to recommend anyone running more than 2-3GB , to use a 64-bit OS. the reason xp 32 wont allow you a full 4GB is because of address mapping of hardware. windows has to account for the addresses of your VRAM,soundcard,etc. and the page file counts as memory addressing.so if you have 4GB of physical memory then you have officially filled your maximum addressing and resources for your hardware and os used and must be accounted for from there. thats why how much physical memory you have left ,may be different from system to system(3.25-3.5GB)
the problem with that is that the truth is, windows xp 32(or any 32-bit os) was not designed with the thought that more than 1-2gb of physical memory was going to be necessary, of course this was years ago when 256mb was good for a gaming system and 512mb was the dream. thats why youll notice if you install windows32 with 1gb,2gb and 3gb, youll see after 2gb the page file no longer changes. but now games and software are starting to tap beyond the 32-bit threshold.
for example..lets say you have winxp 32 and 4gb of whatever ram,..heres how windows sees it
4096mb physical ram
2048-4096mb (default install windows page file w/anything over 2GB) =6GB minimum . 3GB is not an issues because the max addressing hasnt been reached with physical ram. so its a 2gb= games and apps, 1gb for OS split. and resourcing of hardware goes to the page file .
now if you have a 64bit os running a 32-bit game(x86 only) that 32-bit game can only use a maximum of 2048 mb of physical memory because thats all 32-bit applications are allowed,so if your running unreal tournament3 under vista64 with 8gb of memory,....ut3 will be pegged at 2gb and vista64 will be using its usual 900mb-1.1gb so thats a max of 3GB with 5GB for nothing but vista superfetch,. thats why alot of people were disapointed that a 32-bit vista was even made. because it would have helped the pc into an eventually, all 64-bit world.
so if you really want to use more than 2gb of ram effectively,.. you need a 64 bit os running 64bit application's like hellgate london that comes with dx9,dx10,x86 and x64 paths.
primary system-
Core2 E6850 / EVGA 750i SLi-FTW- / Intel Pro 1000PT / 4GB G.skill PQ series / 2xG92 GTS SLi - 2xTT DuOrb / X-Fi Fatal1ty / 150GB WD Raptor / PC P&C Silencer 750(Black) / ANTEC nine hundred
HTPC-
AM2 6000+ / ABIT KN9S / 4GB G.Skill PQ Series / Sapphire Radeon HD2900XT / HT Omega Claro / 250GB Seagate Barracuda / PC P&C Silencer 610