Caveat - If I misunderstand this issue, sorry. My understanding is that installing XP32bit SP3 with 4GB RAM is the best you can do with 4GB physical RAM. There are no other hacks other than running lean as you can with as little crap in the background as possible.
Besides the information below... Physical address extension is enabled by default since SP2 of XP if that is what we are talking about here. The article below explains the whole XP 32bit address limitation of XP 32bit but here is the long and the short of it. Let's say you have 4GB installed physically.
XP 32 can only use a memory address space of 4GB. It then "RESERVES" addresses in that table for loading every little bit and piece in your computer (Devices and so on like a SATA controller... They all need an "address RANGE" to exist in so that the Os can communicate with it) leaving 3.25GB (3402084 in task manager) of RAM free for the system. XP loads up and uses some of that. Typically after XP loads into memory you will have about 2.9GB free. If you have 4GB physically installed that is.
The more RAM on the video card, the less you have free to the OS is the concept because the OS needs to reserve the address space for the RAM on the video card. I have a 512Mb card and I have XP SP3 reporting 3.25GB free. Theory is that if it were 1GB card I would have less reported available to the Os because the addresses for that 1GB will need to be reserved.
If you have a 2GB PC with a 1GB Card and a 32bit XP install. You likely have very little left for the FSX app to run in.
http://www.dansdata.com/askdan00015.htm
"a note about the /3GB, /4GT and /PAE Windows
boot.ini switches, too, because they often come up when people are talking about 4Gb-plus Windows PCs.
They are all useless to you. You do not want them. /3GB and /4GT are config settings for different versions of Windows that tell the operating system to change the partitioning of the 4Gb 32-bit address space so that applications can use 3Gb and the OS
kernel only 1Gb, as opposed to the standard 2Gb-each arrangement.
They don't help at all with the 3Gb barrier, and most applications don't even notice them, so desktop users lose kernel memory space (and system performance) for no actual gain at all. The /PAE boot.ini switch, on NT-descended Windows flavours, activates the
Physical Address Extension mode that's existed in every PC CPU since the Pentium Pro. PAE can also be enabled by
the /NoExecute entry in boot.ini, which turns on support for
the NX bit which you probably also don't actually want.
PAE mode, in its proper form, cranks the memory address space up to 64 gigabytes (two to the power of 36). The computer can then give a 4Gb addressing block within that space - or even more, with extra tricks - to each of several applications. PAE's no good to the everyday 3Gb-problem-afflicted user, though, for two reasons. First, it presents 64-bit addresses to drivers, and thus causes exactly the same compatibility problems as a proper 64-bit operating system. Except worse, because now you need PAE-aware drivers for 32-bit Windows, instead of plain 64-bit drivers for a 64-bit OS. From a normal user's point of view, PAE gives you the incompatibility of a 64-bit operating system when you're still running a 32-bit OS. For this reason,
Microsoft changed the behaviour of the /PAE option in almost all versions of WinXP as of Service Pack 2. They fixed the endless driver problems by, essentially, making /PAE in XP not do anything to addressing any more. All versions of WinXP except for the
x64 Edition now have a hard 4Gb addressing limit, no matter what hardware you use them on and what configuration you choose.
All PAE does in those versions of Windows is activate NoExecute support. Which, once again, you probably don't want.
This isn't a big problem, of course, since XP is not meant to be a server operating system. But it's still mystifying to people who try the /PAE flag and can't figure out why it doesn't work.
Oh, and just in case you for some reason still wanted to try PAE: It eats CPU time, too."
