Same happened to me... in the end I installed the latest bios and tada - 8GB visible in x64 and usable as well - the bios always recognized it though, just didn't work within the OS.
Turned out the bios upgrade switched memory remap to automatic (I didn't see any option before) so this did the trick.
Nothing additional in the boot.ini / boot line either.
Your solution lies within the memory remap, or whatever it's called in your bios.
Good luck man, I know it's frustrating
