XPSP2 doesn't see my memory

[unicorn]

or at least only part of it. Here is what that gives:

Mobo: ASUS A8N-Sli
Processor: AMD 64 3700+
RAM: 4 x Corsair 1024MB
OS: WinXP Pro SP2/WinXP Pro 64

Situation: XP Pro doesn't report all the 4GB of RAM. It claims 3GB. This is reported at different places, for example by right-clicking "My Computer". Frustrating. During the boot sequense it looks as the computer (mobo) works allright. The BIOS reports 4GB RAM.
WinXP Pro 64 reports 4GB.

Done: I have tried a few different switches in boot.ini, including "/PAE". I have checked the RAM modules one by one and they all report 1024 MB. I have checked the M$ database without finding the page describing what I should do. However I have found that XP Pro is supposed to supposed up to 4GB RAM memory...

Suggestions or even known solutions are freak'n welcome!