Unity 5 has not even been released yet, it's unlikely we'll switch before that.
But, yes, do want 64 bit.
EDIT: Though I should point out that the memory constraints of Unity have basically nothing to do with changing the size of our maps, that isn't what limits them.
EDIT 2: Also, Keyvias is a patch or two behind. Guns 1.3.7 was released on Unity 4.3.3, Guns 1.3.8 will use Unity 4.5.4