For 2009-2012 Mac Pro, MPG recommends going to 48GB for 4/6 core machines, and 96GB for 8/12 core machines, rather than mixing and matching.
Due to some unusual circumstances, OWC memory for Mac Pro has dropped a lot in price, but this is likely to be a short-term thing:
Not everyone needs 8TB hard drives which MPG is soon to test.
Why buy drives at OWC? OWC adds-on a 90-day *replacement* warranty, meaning that a problem drive will be replaced with a NEW drive within 90 days. This is a BIG DEAL if you are making a RAID or depending on the drives; sending a drive in for warranty essentially means another one has to be purchased to fill the hole—not so with this OWC 90-day guarantee.