The limit is also affected by the controlling chipset.  Galadriel has triple channel single slots with 4GB sticks.  I asked Alienware about replacing those with 8GB sticks;  the tech said that the chipset can only read 12GB.  They may have a 24GB board next year, though.

You'd probably need to look at workstation motherboards.  Cameron has triple channel RAM with four slots per channel;  filled with 8GB sticks, that's 96GB.

Dell has a midrange  Vostro and Optiplex workstation series which tops out at 24GB, if I remember correctly.  At the top of their food chain is the Precision workstation series with models numbers like T1600 through T7500.

Poser 12, in feet.  

OSes:  Win7Prox64, Win7Ultx64

Silo Pro 2.5.6 64bit, Vue Infinite 2014.7, Genetica 4.0 Studio, UV Mapper Pro, UV Layout Pro, PhotoImpact X3, GIF Animator 5