How much RAM do you want/need? You can get 3x16GB modules, "unofficially". If going with 8GB modules, I would say you want to keep in pairs - either 2x8GB or 4x8GB.
I know these work:
http://www.amazon.com/Kingston-ValueRAM-PC3-10666-KVR13R9D4-16/dp/B008542F60
1333MHz DDR3 PC3-10666 ECC at 1.5V
^^^^Hard to believe that a 5,1 would work better with four sticks than three? The fourth slot is electrically tied to the third slot. If the third memory slot is left MT, a stick in the 4th slot will not be recognized.
From personal experience, I have a dual CPU 5,1 with 6 of the eight slots populated and no RAM issues with quality DMS RAM.
Lou