Yo, Scooter!
The Mac Pro can employ 32 Gb in eight slots.
It is proven to work better with at least 8Gb.
The price of denser Ram is always a premium, especially at the beginning of the purchasing cycle - like now. I don't think 4Gb chips result in a quicker machine than 2Gb chips.
I bought 8x2Gb, discarded the 2x1Gb that came with and all the slots are full. I decided this was optimal since by the time I can exploit more ram than that, it will be time for a new machine. $849.
If you have the dream of more than 16Gb ram, then pay the premium and get the dense ram with an eye to filling the slots later with less expensive chips. It is a hefty premium right now. You might consider 8x2Gb and resell them later when 4Gb chips descend. You could be ahead financially that way.
BTW: Extreme easy install on hard drives and RAM - no need to be scared of after market add-ons. They made the hard drive slots so your grandma could do it with a soup spoon and a phillips head screw driver.
BTW: Don't mix OWC RAM with the Apple RAM. The OWC RAM ridicules the Apple RAM when there is nothing else to do, like at night.
"Hey, overpriced Apple RAM. Youse only gots eight fins while we gots 14. We way cooler than you."
Dwight Atkinson