8Gb of ram on an MSI X-48

I cant seem to get my board to boot with 8Gb installed....
all the ram tests fine , so I know its not the problem.
I can run 2 sticks without issue, but no go when I put in all 4 sticks...
I'm running a Q9650 @ 3.6.. 9X400... Vista ultimate ...its PC6400 ram....
all settings stock except CPU voltage....what should I be looking for as far as changing anything
Thanks in advance...

Try increasing the NB Voltage from the default of 1.25v to 1.3 - 1.33v. Also increase FSB Terminator Voltage (VTT) to 1.30v, and CPU Reference Voltage to 0.82v - this being 63% of VTT.

OK, thanks for the reply..Ill give it a shot..