I'm a little iffy on the true benefits of 64-bit computing but here's what I do know:
1. The big benefit, as far as "normal" computing goes is that the computer is able to address more than 4GB RAM. 64-bits represents the size of the registers on the processor and, as such, the biggest number that the computer can use is 64-bits long. 32-bit registers means that the biggest number the computer can understand is (I think) 4,294,967,296 and therefore memory addresses in RAM must be equal to or less than this number to be accessible. 64-bits gives a maximum number size of 18,446,744,074,000,000,000 and that effectively removes any practical memory limit. OS X, as you may be aware, loves RAM so the more you can throw at it, within reason, the better so 64-bit computing will help you out there.
2. Less beneficial for normal users but great for the scientific community is that 64-bit computers can handle bigger numbers with greater accuracy. My theory is a little iffy here but since your computer can handle really big numbers (beyond the limit imposed by 32-bit registers) it has to "fake" the ones bigger than the 32-bit limit which effectively reduces accuracy. 64-bits allows much bigger "real" number so scientific applications can be much more accurate in the higher ranges of numbers.
3. Big databases, I believe, appreciate 64-bits for reasons similar to #2. I am not 100% sure about this...
Beyond this I am not sure of the benefits of 64-bits. Some of the 64-bit processors run faster than the 32-bit ones but I am unsure as to whether that is as a direct result of 64-bits or just the opportunity to redesign the processors. The overall benefit for "regular" users is just access to more RAM and possibly processors with an improved architecture. Beyond this 64-bit computing means pretty much nothing and it won't deliver, for the sake of examples that I've heard, better Artificial Intelligence in games or other such nonsense. 128-bit computing simply isn't necessary at this time.