Installed 4GB memory but Windows XP 32 bit only reporting 2GB?

I've just taken an existing XP Pro 32 bit system that had only 0.5GB of memory installed and maxed it out to 4GB.

The BIOS reports the 4GB ram however when XP is booted and I look at the computer properties only 2GB of RAM is reported. Can anyone explain this?

Before we go up any blind allys the /3GB switch is not the answer here, I have no need for a single process to use more the 2GB of memory.

I'm wondering if the the 32 bit XP Pro is deliberately limited to 2GB. I seem to remember seeing an excellent table on a Microsoft site listing all the various SKUs of Windows and what each one was limited to. However I can't seem to find that table now.

Edit: Motherboard info.

The motherboard is a Asus A8N-VM. According to the manual it will accept up to 4GB of mem.

Edit 2:

Well it would seem I've also got a similar problem as reported by this question Windows only sees 768 MB RAM out of 4 GB. In my case the PCI Bus seems to have allocated memory from 0x7800000 all the way to the top, even though the first device inside the connection is the NVidia 6100 starting a 0xD0000000. The difference is the missing 1GB or so.

I have no idea how to convince the PCI Bus to not start its allocation so early.