You have FBDIMMs. They run hot and use more matts.
The reason for a utilty is to know.
Apple specs for the machine
General specs for FBDIMMs
The 2008 Mac Pro 800MHz 1600MHz fsb is best served with all 8 DIMM slots filled
2006-7 is best to have set of 4 or set of 8
A1:2 and B1:2 would be where the 4 x 4GB are best served.
FBDIMMs should be kept below 74*C even though the specs say higher.
If you can keep them in the 68*C range fine. One way, to run SmcFanControl to boost the fans slights.
You can also check Crucial/Micron for Mac Pro memory and any links to the knowledge base as well as others.