The game logic can't be multithreaded because of how Clausewitz is coded. The game wouldn't benefit from 64 bit as all Paradox games are less than 4GB big for the entire game.
A lot of misinformation in this thread.

Both Clausewitz and CK2 are multithreaded, so that is already a fact. Our games will most likely become 64-bit executables in the future, when there is a need to do so. Going from 32-bit to 64-bit has a number of advantages and disadvantages, only one of which is the larger memory address space. I can't speak for what's going to happen in the future, but our current games don't need to be 64-bit yet.