It should say the right pair combo on the computer's documentation.
Otherwise this would be my winning combo:
Mac Pro FB-DIMMs
The Mac Pro Xeon machines use FB-DIMMs. They are installed in pairs. The machine has 2 riser cards that take the RAM, each card has 4 sockets. You populate the RAM in pairs, with the largest RAM in the numerically lowest slots -- in this order:
The sockets 3 and 4 have a higher latency than sockets 1 and 2, so put your largest RAM in the 1 and 2 sockets.
There is a small speed benefit to installing 4 identical modules across both risers: A1 A2 B1 B2 which permits 256-bit access mode. However, do not buy smaller RAM modules just to match an existing pair.
There is a diagram on the inside of 2007 Mac Pros to help identify installation order. Newer Mac Pros have a diagram inside directing you to see the User Manual for installation instructions, instead.
The 2008 Penryn MacPros deliver with RAM in A1 B1. This must be changed if you are adding RAM. If you are adding one pair, put the new RAM in A1 A2 and relocate the Apple RAM to B1 B2.
Hope this helps,