There are two issues with 32/64 bit that need to be considered.
Computation speed is one of these. 64 bit systems are considerably faster at doing computationally intensive tasks. Calculating prime numbers would be an example. As long as you have enough memory to do the task without going into swap it doesn't matter how much RAM you have.
Memory recognition and utilization is another issue. Without the PAE kernel patches, most 32 bit systems can only recognize and use a little over 3 Gig of memory. If you are doing something that is memory intensive then adding RAM and going to 64 bit can be helpful. As 64 bit will enable you to utilize more memory.
One thing to keep in mind however is that a 64 bit system will use between 15% and 30% more memory for a given work load than 32 bit will. For this reason it is pretty much useless to run 64 bit with 4 Gig of RAM just to be able to use the last Gig of RAM. You haven't gained anything, memory wise. If you only have 3 Gig and start going into swap with 64 bit but don't with 32 bit you have actually hurt yourself with computation speed as well.
To really answer your question you would have to know what your usage patterns are. Do you need to boost your computation speed? Do you have enough RAM to support 64 bit computing on your work load without going into swap? Is you task memory bound and do you have enough memory to support a gain by using 64 bit or will the addition memory requirements of 64 bit actually hurt you?
Questions I can't really answer.