Carol: XP doesn't have problems supporting PAE, it simply imposes artificial limitations on how much it supports, for market segmenting reasons. Would be interesting testing a pre-SP2 XP and see if it supports more RAM, because according to the MS link pre-SP2 had the limit physical memory, not address space.
But yes, ASUS is right that you need chipset+bios combo that supports memory remapping, because of memory mapped devices.