there could be several reasons why this is happening. I have come across this kind of problem several times before. i will try to explain.
once i had a board what worked well until all 4 DIMM were populated. So i called up Tech support and they stated that the board Will not work with 4 DIMMs populated as the chipset was not designed to run that way.
Looking to see that you have
5N32-SLI SE Deluxe, i looked in to it and this is probably not a chipset problem. your board should RUN with 4 DIMMs populated.
so this leads me on to believe that your bios is not capable of handing 4 DIMMS populated. So i would suggest that you do a BIOS Flash/Update to the latest bios available, as that is what fixed it for me.
Now i see an update saying you tried that. seeing this i believe makes me think back to the chipset idea.
now since you have x64 bit installed, i don't see why it's not working, but try this:
Go into the bios and see what "Memory remap feature" is set to. if it's disabled set it to enabled. and if enabled try disabled.
then change your memory settings to AUTO. if that doesn't work, do the same thing but try the memory settings at their Native speed, in your case 800 Mhz
but i think it should work on Memory remap feature at: Enabled and memory settings at auto