I am not the most up-to-date guy on the block, but from the computers on which I have worked, what you describe is not possible... The motherboard detects the memory, but the operating sytem uses it. It is true that Windows 7 will use 4 GB or 8 GB, whereas Windows XP, depending on the motherboard, cannot use more than 3 GB, or on some motherboards, 3.5 GB... However, yours is clearly limited to 2 GB.
Perhaps somebody more technically advanced on memory tweaking can explain how the same computer detection could be as wide as you describe.