Yes, that's true. I have been trying to downgrade my A916NR to XP all week and I found a lot of info about it, while trying to harvest XP drivers for the $#%! Vista machine.
It definitely tops at 2GB.
From the specs on Compaq (HP) website it says "Memory: Up to 2GB DDR2". They don't bother telling you the speed. Its a very high level "Spec" view.
The link is
http://h10025.www1.hp.com/ewfrf/wc/document?docname=c01296312&cc=us&lc=en&dlc=en&product=3646594
Here, they at least give you the speed. From the (Downloadable) User Manual it states:
Memory
2 customer-accessible/upgradable memory module slots
Supports dual-channel memory
Supports up to 2 GB of system RAM
PC2-5300, 667-MHz, DDR2
It is good to know what would happen if I over-upgaded the memory. But, there really is no solution to your problem, unfortunately. The vendor should, however, exchange the memory for you.
I just ran across your post looking for XP Driver Support so I'll probably not be back here to followup. Good luck.