Questions on Adding Memory to a R420

Dell PE R420

2 Intel Xeon E5-2407 2.20GHz Processors with 24 GB RAM - (6) 4GB RDIMM 1333 MT/s, Low Volt, Single Rank modules.

The server has 12 DIMM slots.  
I have not opened the server.

I am guessing that the RAM is set with 3 cards for each CPU in chan 1,2,3 on the first slot of each channel (white tabs)?.

I want to add an additional RAM to the system.  
What are the options to add an additional:

16 GB
20 GB
or
24 GB

I am assuming that adding the cards to the slots and channels will depend on the type of add-on RAM
purchased and that each CPU should have the same amount (i.e. 24GB on each in some combination).

I have checked a  few options

From Dell Site:
A6996808 - 8 GB Certified Replacement Memory 2Rx4 RDIMM 1333MHz LV
A6994455 – 8 GB Certified Replacement Memory Module 2Rx4 RDIMM 1600MHz LV
A6994474 – 4 GB Certified Replacement Memory Module 2Rx8 RDIMM 1600MHz LV

From Crucial
http://www.crucial.com/store/mpartspecs.aspx?mtbpoid=D4BF3CBCA5CA7304

Any help would be appreciated..

thanks