Windows XP, 4 GB RAM shows only 2.7 GB

I got here a computer with 4 GB of RAM installed. All 4 GB of RAM will also appear in the BIOS. But under windows xp it shows "only" 2.7 GB ram. Any suggestions?
Asus A8N-SLI Premium
AMD Athlon 64 X2, 2000 MHz (10 x 200) 3800 +
BIOS Type Award

See suggestions given in similar type of threads: http://forums.techarena.in/xp-hardware/1084699.htm
http://forums.techarena.in/xp-hardware/954251.htm

If you have Windows XP with the 64 bit version installed, then the memory will be fully recognized. In a Windows XP 32 bit it will not show more that 2-3 gb of ram.

Asus writes:
If you are a total of 4GB of memory installed, the system is due to address space allocation for other critical functions detect less than 4GB:

- System BIOS (including motherboard, expansion cards, etc.)
- Motherboard resources
- Dump I / O
- Configuration for AGP / PCI-Ex / PCI
- Other memory allocations for PCI devices

Different onboard devices and different add-on cards (devices) are a different total memory size required.
Example: More installed PCI cards, more memory and thus is less memory for other use.

On a SLI system, since PCI-Ex graphic cards occupy about 256MB, 256MB, further demonstrated when a second PCI-Ex graphic card installed. Consequently remain but one of only 2,75 GB of memory if two SLI cards on an A8N-SLI Premium installed, while 3.0 GB of memory with only a graphics card with no other extension devices are available.

This limitation applies to most chipsets & Windows XP 32-bit operating system.

If you are running Windows XP 32-bit version installed, we recommend that you install less than 3GB of memory. If your system requires more than 3GB, following 2 conditions must be met:
1. The memory controller which supports memory swap functionality is used. The latest chipsets like Intel 975X, 955X, Nvidia NF4 SLI Intel Edition, Nvidia NF4 SLI X16, and AMD K8-processor architecture to support the memory swap function.
2. Windows XP Pro x64 Edition. (64-bit) or other operating system which is more than 4GB of memory can be addressed.

There are people that have coaxed XP into seeing up to 3.5GB on 4GB systems. Some claim it's due to hardware issues and some motherboards will allow XP to address all 4GBs.

There's some interesting reading here:
http://www.howtofixcomputers.com/bb/ftopic117376.html

It seems as if part of it is hardware limitations, but 32-bit XP has some limitations of it's own.