Use this guide to upgrade the RAM in your 2009 model Mac Pro (8-core) to a maximum of 96GB or 128GB, depending on if you have a W, E, or X series CPU.
According to this post on the macrumors forum, MacPro4,1 models with a 'W' or 'E' series CPU can only utilise up to 96GB on 8-core models. Only those with an 'X' series CPU, which is the 8-core 2.66GHz or 8-core 2.93GHz models, can utilise up to 128GB of RAM. (16GB x8).
If you try installing 128GB (16GB x8) on a 2009 Mac Pro that does not have an X series CPU then the memory in the fourth and eighth socket won't be recognised.