Woah. That article is extremely misleading.
Here's the real deal.
To make use of 4 GB or more RAM in a 32 bit OS you have to have:
- a BIOS that successfully remaps the space from 3-4 GB to 4-5 GB (because that space is taken by devices and the RAM there needs to be remapped).
- working PAE to make use of the memory above 4 GB (independently of whether you have more than 4 GB RAM or whether you have remapped memory of your 4GB kit living above 4GB addresses).
That's all. It is true that Mickeysoft has decided to by default disable working PAE in XP SP2, but it did have working PAE in before SP2, and other 32 bit Windozes like Win2K also have working PAE. Needless to say, all of Linux, FreeBSD and NetBSD have working PAE.
Since I don't use it I haven't tried first-hand, but I have been told that manually activating PAE in SP2 works fine, except that many many drivers are written violating Microsoft's requirements (which include proper support of PAE).
If you miss the proper remapping support (point 2), then you will permanently lose memory that is from 3-4 GB, how much depends on what PCI/AGP/PCIe cards exactly you have installed. Example: you have 4 GB, no working remapping and 724 MB worth of devices the OS wants reserved, then you have 3.3 GB usage. You have 6 GB RAM, no working remapping and 1 GB worth of devices, then you get 5 GB (you lose the space between 3-4 GB). If you have 6 GB and working remapping, and 1 GB worth of devices, then you get 6 GB - because the remapping now has memory living at 0-3.0 GB and 4.0-7.0 GB, because the memory in the space between 3-4.0 GB (the space used by devices), has now not been deactivated, but remapped.
Now, all the above is about physical memory, aka RAM. In addition there are limitations in virtual memory o 32 bit OSes, which manifest themselves in how much virtual memory each userlevel process can take, but mapping X MB of virtual memory still can mean much less, or even much more, actual physical memory (RAM). But that's not the topic here.