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 |