You can increase your computer memory by installing memory modules on the system board. If you incorrectly replace or install a memory module in your computer, it will not boot. There are no memory size limitations between the DIMM A and DIMM B sockets. However, if you add memory to the DIMM C or DIMM D socket, special configuration requirements are needed: If you add memory to the DIMM C socket, the memory size must match the memory size in the DIMM B socket. If you add memory to the DIMM D socket, the memory size must match the memory size in the DIMM A socket. For example, if you increase DIMM D to 4 GB, you must also increase DIMM A to 4 GB. Note that the DIMM A socket must always be populated
Solved! I know this is an older post but maybe I can help some others that are having this issue. I did the same thing as spaceodyssey. I had two 2 gig sticks in slots A and B and purchased two 4 gig sticks. The computer would not boot with all four sticks in place. It would work fine with either pair alone, just not all four together.
It just makes sense that you would put one matched pair in slots A and B and the other matched pair in slots C and D. The info from DELL-Nikhil D supported that. The rank on the new pair is 2Rx8 and the original pair is 1Rx8. I thought that might have been the problem. I was getting ready to buy some different RAM.
I had messed with this and researched it for days till I came across this instruction in the Setup and Quick Reference Guide: "When upgrading or replacing memory modules, the modules must be paired and matched in slots B and C, and in slots A and D. Otherwise, the computer does not boot."
I put one matched pair in A and D and the other pair in B and C and it's all working together just fine. That fixed it! The manual is on the Dell website here: http://www.dell.com/support/home/us/en/04/product-support/product/precision-m6400/manuals
The information in this forum from DELL-Nikhil D about populating RAM slots on the M6400 did not work for my situation and I believe is incorrect as it contradicts the Dell guide quoted here.