Why wont xp 64 bit recognize over 3gb ram?

Welcome to Tom's Forum! :)

There are several possibilities:
1. MOBO limitation
2. Maximum Memory is set in MSCONFIG -> http://i1013.photobucket.com/albums/af254/Jaquith/Boot_OPT.gif
3. BIOS Memory Remapping -> {is not} Enabled {Enable for x64 OS}
4. Bad/Incompatible/Mixed RAM, Bad DIMM Slot(s), Wrong DIMM Slots used, etc...........

Q - What MOBO?
Q - What RAM? - need links to RAM
Q - What DIMM slots, exactly, has what RAM - Example: CPU | 2GB | blank | 2GB | blank |
I cannot find the "asus pgd2 deluxe". Is it from an OEM PC e.g. HP, Dell, etc? If it is then I'd a link.

Keep in mind if this is an older MOBO/PC then many of the LGA 775 were limited to 4GB with 3.0GB~3.5GB 'usable' regardless of the amount of RAM or OS.

IF your BIOS lacks Memory Remapping then 3.0GB~3.5GB might be your limit, but I still need to figure-out what you've got before assuming.'

Further, some MOBO's require the Channels to be EQUAL in size AND the RAM density per DIMM slot not exceeded:

CPU | 2GB | blank | 2GB | blank |
CPU | 2GB | 2GB | 2GB | 2GB |
CPU | 2GB | 1GB | 2GB | 1GB |