Is it possible to install 24 GB RAM by using all 12 RAM slots (2GB sticks each) on a dual socket mainboard even though only 1 CPU is installed? (this is on a Dell R710).
The Dell R710 does not support this type of configuration - the DIMM banks are per CPU - with 9 DIMM slots per socket ( three DIMMs per channel, three channels per CPU) for a total of 18 DIMM slots. If you install DIMMs in the slots for the second CPU they cannot be accessed by the primary. More specific DIMM configuration details can be found here. For a single CPU system the maximum RAM is 96GB but you cannot use more than 9 DIMMS in total to get to that number. Just as a reminder, if you do fully populate all DIMM slots in a channel you are going to be limited to 800Mhz memory speed.
The Dell R810 has a slightly different architecture that Dell call FlexMem Bridge (described in this Anandtech Review) that does support populating all DIMM slots even when only one CPU is installed but that is a very different system.