Windows 7 x64 will not boot with memory upgrade (P6N SLI Platinum)

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Windows 7 x64 will not boot with memory upgrade (P6N SLI Platinum)
I recently bought 4x 2GB sticks of Crucial DDR2 memory.  I originally had Windows 7 x86 installed, and with all four sticks in place there was no problem (except the memory limit for x86).  All 8GB of memory registered fine though. 
I decided to install the x64 version, but setup would hang at every step and installation took over 12 hours - Windows 7 would not boot either when it was complete.  I removed all but one of the 2GB sticks and everything installed fine and it's up and running now.  If I put the rest of the sticks back in, Windows begins to load but gets stuck at a black screen.  I've swapped each 2GB stick in my system and they all work as long as only one is inserted at a time. 
MSI P6N Platinum SLI (v1.7 BIOS)
Windows 7 x64 Ultimate
Intel Core 2 Quad @2.4GHz
4x 2Gb Crucial DDR2 @ 800 MHz
NVidia GeForce 8800 GTS 640Mb
I've done some google searching, and it seems like other people are having the exact same problem with the P6N boards, but I've never seen any solution...
Repaly
The memorycontroller on that board is an oldie, got one myself  and it will probably not run @800MHz with all slots occupied on a x64 OS.
That contoller is only rated for 667MHz when running all channells fully occupied.
The fact that you could run it so under x86 has probably to do with the fact it didnt adress all the memory, but only 3.25GB max and thus it was stable enough.
Now you are running an OS that uses all memory and thus it can become unstable while its running at a too high speed for the controller.
I had the same and could "only" get the memory to work @750MHz tops, before it decided to BSOD or bootloop on me.
This is inherent to its design (from NVidia, they made the chipset) and cannot be overcome, but dont worrie yourself, the speeddifference between 667MHz and 800MHz is negligible and will only show up if you run benches, in normal day to day use, you will hardly, if ever, notice it.
But, do you really need 8GB???
If you do gaming, internet, office and other "light" work, i.e. normal homeusage, 4GB is already more then enough.
8GB is nice if you use cad/cam, photoshop and other memory intensive stuff.
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