Actually, the 64-bit version should really run faster, since rendering is mainly a CPU-intensive application, and this usually outweighs the larger amount of data to be moved. I am not sure for Poser, since i only have the 32 bit version, but in my experience with povray on my computer the 64-Bit version renders about 15% faster  than the 32-bit version, and i am guessing the poser renderer is doing more or less the same thing (essentially producing colored pixels from some geometry).
If you know of some speed comparisons 32 vs 64 bit i would like to know, since i am pondering about acquiring the 64-version myself.