I just purchased 4x2GB of RAM to add to my 4x1GB of stock RAM in a 2008 Mac Pro 3,1 with Mavericks 10.9.1. I have 2x2GB in slots 3 and 4 and 2x1GB in slots 1 and 2 for a total of 6GB paired on both upper and lower cards. System Profile shows 10GB of Memory instead of the expected 12GB. When I incorrectly had 4x2GB on top and 4x1GB on bottom, it showed 8GB total, so I am making progress, but how should they be ideally installed to get all 12GB of available usage? BTW, this machine is also running Mavericks Server.