Since you are talking about 6 sticks, I presume you have a X59 platform, correct?
In that case, make sure you have three identical sticks in each bank, for instance 3 x 4 GB and 3 x 8 GB (if 8 GB sticks are supported, which is rarely the case). Do not mix different sticks in one memory bank.
Actually, the OP is going to leave out two of the 4GB sticks. Therefore, going with only four sticks total - 2 x 8GB plus 2 x 4GB.
In this case, the OP must have had either an LGA 1155 build or an LGA 2011 build (the older LGA 1156 platform does not officially support 8GB sticks). In the case of LGA 1155, dual-channel operation is still in effect, depending on which slots the particular sticks are installed in. (In this case, performance in CS6 will be a bit faster with 24GB than with 16GB but not as fast as with 32GB.) But on an LGA 2011 platform, memory controller performance will be degraded: Depending on the BIOS of the LGA 2011 motherboard in question, the memory controller will either default to the Flex memory controller mode (in which 16GB will run in quad-channel and the remaining 8GB will run in single-channel) or entirely to the single-channel mode.
Since 4GB and 8GB have different timings I would not recommend this configuration. That will force the controller in the CPU to moderate the differences and right now and board's do not have the best timing detection to sort that out. The suggestion is to run just the 8GB for now.