I don't expect to get the memory back, I just want to know where it went
Same place it went when you had 3071MB, to hardware resources. You changed hardware, thus have different hardware resource requirements.
It has nothing to do with 32-bit XP or Vista only being able to use 3GB, 2.8GB, or any other number that is less than 4GB. 32-bit XP and Vista will use as much RAM as the BIOS exposes to it, up to the 4GB limit. It so happens that the BIOS cannot expose all 4GB to 32-bit Windows XP or Vista, because the hardware must utilize some of the 32-bit (4GB) total physical address space, or the system would be an expensive door stop.
How much the BIOS must reserve to hardware (thus cannot give to RAM addressing) depends entirely on the hardware configuration, industry specifications, and to a lesser extent, the BIOS writer's design choices.