Use this guide to upgrade the RAM in your 2010 model Mac Pro (8-core) to a maximum of 96GB (16GB x6). Note, although this machine has 8 memory sockets, it can't utilise any more than 96GB of RAM.
According to this post on the macrumors forum, MacPro4,1 / 5,1 models with a 'W' or 'E' series CPU can only utilise up to 48GB on quad-core models (single cpu) and 96GB on 8-core models (dual cpu). Those with an 'X' series CPU, which is the 8/12-core 2.66GHz or 8/12-core 2.93GHz models, can utilise up to 128GB of RAM. (16GB x8).
Note that 16GB ECC registered modules cannot be mixed with ECC unbuffered modules of other capacities (4GB/8GB)
If you try installing 128GB (16GB x8) then the memory in the fourth and eighth socket won't be recognised.