The Mac Pro (Mid 2010) supports 8GB registered DIMMs (R-DIMMs) for up to 64GB of memory in 8-Core and 12-Core computers, and up to 32GB of memory in Quad-Core and 6-Core systems.
Do not mix R-DIMMs with unregistered DIMMs. If you install both types of DIMMs in the same Mac Pro, it will not start up.