Won't boot with 8GB RAM

I have a HP/Compaq dc7800p small form factor PC.  I am trying to upgrade the RAM to 8GB.  I have the correct kind of RAM (it is Samsung and matched set of 4 x 2GB).  When I fill all 4 slots, I can't get the PC to boot.  It just sits there for 30 sec, and then the system fan goes into overdrive.  I have tried every possible permutation of putting the modules in the slots, but the most the machine will let me add is 6GB.  I know the primary slot has to be filled, but I can't figure out what is the problem. 

Thanks in advance for any help.

CeeBee