
Memory Type: DDR3 PC3-8500, DDR3 (ECC)
Maximum Memory: 96GB
Slots: 12
Each memory slot can hold DDR3 PC3-8500 with a maximum of 4GB per slot.*
*Not to exceed manufacturer supported memory.

You cannot mix Registered and Unbuffered memory. If you use the Crucial 8GB modules (CT1530950) or the 24GB Kit (CT1530951) all previous existing memory must be removed.
96GB maximum memory supported using Mac OS X Server v10.6

Memory or DRAM is the 'working' memory of the computer. It's used to store data for programs (sequences of instructions) on a temporary basis.

video - installing memory