Wappleg8,  That is correct. The key to the population of the dimms is that the dimm channels you described must be identically matched. So if you have 4 4gb dimms in the white tabbed slots then you can place the new 4 in black tabbed slots just adjacent to the older dimms.  Let me know if this helps answer your questions.
I see that the YM-158  motherboard has four banks of three RAM slots. I currently have the first slot of all four banks filled with crucial CT51272af667 4GB RAM (total 16GM RAM). I'd like to upgrade the RAM, and am curious to know if installing 4 more crucial CT51272af667 4GB RAM in the second slot of all four banks would be the best way to do this.  Any input would be much appreciated.  ~Bill
DPE2900 RAM Upgrade