I wish my Leopard install would be able to use all 8GB I have. I installed 8GB and it wouldn't boot all the way to the desktop...just blue screen with no icons or cursor. Removed a stick of DDR2 and it would boot to the desktop with 6GB installed. About this Mac only recognized 4GB, but System profiler identified all 3-sticks (6GB). Apps like console, software update and the installer app would launch and quickly crash. Removing another stick (4GB total) would give me a more stable system with all apps functioning as they should.
I guess it's an issue with AMD processors and 64-bit addressing under Leopard (or lack there of).
Perhaps your motherboard only support 4GB or the BIOS is configured to support only 4GB.