The only way to actually use the 64 bit parts of a 64 bit CPU is to run a 64 bit version of the operating system, otherwise the CPU is just a faster 32 bit version (quite a few people run it that way though). Run a 64 bit linux or the 64 bit version of Windows (not that I would recommend using the 64 bit version of Windows, it’s missing too much atm) and you could run a 64 bit build of Blender/Yafray, which may or may not be faster, then an 64 bit optimized version of them could be created, though I’m not sure how much optimization would be applicable, outside of access to more memory.
You cannot, however, just use 64 bit code from 32 bit windows/linux, which is what a 64 bit renderer would most likely be.
It probably would’nt be a bad idea for somebody to build a 64 bit blender and test it’s performace and if they are a coder familiar with 64 bit processing look for possible improvements, but currently it’s just not a point in time to be working with 64 bit, it’s still a little on the immature side. (I have an Athlon 64, some I’m definatly not just trashing it unjustifingly)