I have a similar machine and it's a chipset limitation. The old chipset can't map peripherals beyond the 4GB boundary so you have to leave some open space for the video and other devices.
Amilo 2727 will let me upgrade ram to 3 gb (2gb+1gb) not 4gb (2x2gb). Why? I read online that its possible to upgrade 4gb but when i try this it will not load. I have windows 7, 64 bit and 2x2gb 200 Pin SO DIMM DDR2 PC2-5300 667MHz. Please help