Well the PAE switch is enabled by default I thought, at least in XP Pro SP2. I believe the indication is when you right-click on 'My Computer' and you see "Physical Address Extension" at the bottom of the 'General' Tab. I know your CPU, OS, and motherboard have to support it. We know you're CPU and OS support it, and I would assume from the 'Memory Remapping' option you can enable in your bios that your motherboard should support it.
I would check to see how much RAM XP can see with 'Memory Remapping' turned off. If you can still only see 2GB then you have to much crap loaded into the memory address space by your BIOS. Do you use onboard video? you might have to disable any onboard crap that you don't use because anything MMIO will decrease the address spacing available to your RAM, effectively giving you less RAM. Even discrete graphics card memory is mapped into the memory address spacing so if you have like 2x512MB or 2x1GB video cards then that'll compound the issue too. Maybe that is the issue and your memory remapping on your board just isn't working. If you can't get more memory seen by XP by disabling onboard stuff then I would call ASUS as it would seem your board is not really 'Remapping' memory at all.
Arson94