Except for the fact that WoW + TBC + WotLK comes to about 11Gb or so.
I'm running with 12Gb of DDR3 RAM, coming from 4Gb of DDR2 RAM and I can run 5 accounts through Dalaran on 2x22Inch monitors.
I don't think 12->16 is worth it but 8->16Gb might. Depends on the specs of the rest of your system.