Where would the Motherboard come from? What about the Operating System needed? From the research I did, which is limited, there is not an OS today for that much RAM. I would venture the answer is no today to build one. Maybe I am wrong. The processor architecture today I do not know if there are limitations on RAM with 64bit systems. Maybe there isn't. I know with 32bit OS it is 4GB of RAM is all that can be used. Any more is a waste of money.
Sticking to RAM the greatest stick I can find available for a personal PC and not a server is 32GB. So, with four slots, which is the most common and greatest for a motherboard, that means 128GB RAM. That is the greatest I found researching available motherboards stating that as a MAX for RAM.
A quote I saw shares, "the current largest stick of RAM I can find on one particular online retailer is 32GB. It would take 32 of these to reach 1 terabyte. At about a half inch per stick this brings us to a devoted 16 inches of space on your motherboard for a terabyte of commercial ram."
So, Corsair Vengence stick for 32GB is about $200. That means RAM alone would cost $6,400. Everything else using the top of the line probably arrives near $3,000. Bottom line appears to be 10K with a little hot rod stuff like water cooler and such.