You cant mix 2GB sticks with 1GB sticks in a MacPro? I got an 800Mhz mac pro and the stock 2GB (1GBx2) but it isn't recognized by the system (after installing the new 2GB modules). The 4x2GB (8GB total) is recognized that I'd bought from newegg. Originally when I had the 1GB modules in slot 1 then the rest in slots 2,3 on both boards it recognized 6GB available. The 2GB are all FB dimms they work just not the 1GB's that came with it. I now have all 8GB spread across slots 1,2 on both board and the 1GB modules occupy slots 3 on both ram cards.