My Win7 7201 sees and uses all my 8 Gbs of Ram alright, as well as my Vista x 64 does. As it has been suggested, you should check if your bios detects your total amount of Ram, watch at the very start for POST to detect and show the number of the total amount of Ram installed. Then see if xp x64 or vista x64 detects it alright.
In other forums I have come across users reporting the same issue. So, I don't know really, could this be dependent on one's hardware or one's 7 build ?