I have been pondering the same question myself, and afraid I wont be much help... However, I will tell you my own plan and perhaps that can help.
- my plan is to use a motherboard/processor (i7-5820k/ Asus x99a) that can handle 128gb of ram EVENTUALLY
- then I can start with 64 gb of RAM, eventually upgrading to 128 gb
BUT, if by the time I want 128gb, several years (aka centuries in technology-speak) have passed and it makes sense to buy a new computer, I wont have wasted the money on 128gb
Also, the other trick is to just pick the 5 organs you want the most... are any of them over 64gb? If they are, can you handle trimming some of the organ so it fits on your available ram? VERY FEW organs need 128gb.
Also, if youre not using surround sound, you will NEVER need 128gb.
Hope I was helpful! If not, it was at least helpful for me to summarize my own thoughts for myself as I am about to purchase my next computer