Q: When all four memory slots are filled with 1GB of DIMM each, the system will only show 3200 MB of memory instead of 4096MB. Whichever one of the modules is removed, the system will show 3072MB as it should. How could I get 4096MB showed by system with 1GB*4 DIMM installed?
A: If you install four 1GB memory modules on P5GD2 series (or all the other models), the system may only detect a size between 3GB and 4GB, because of memory allocation for the devices in your system per PCI specificaton
If the system can only address 4gb of RAM, and 800meg is reserved by the system, then putting in 4gig is a waste.
Something similar (actually, I think it's the same) to the original 8088/8086's 640k/384k split - 384k reserved by the system for devices leaving 640k for user programs. Besides, no one will ever need more than 640k...