I don't think that matters as on a MB wouldn't its kernel in Snow Leopard automatically be 32-bit? There are a few threads stating that only certain models of Macs will actually support 64-bit kernels??? And, most Macs will be able to run 64-bit apps with 32-bit kernel. And memory allocation doesn't depend on kernel but rather on apps and CPU...
I don't really know for sure, but I do know that Snow Leopard requires 1 GB RAM whereas Leopard only required 512 MB... However, I think that Snow Leopard would better utilize it with 64-bit apps.
Back to the OP>>> it depends. Honestly, I doubt you're using your full 2 GB of RAM. Snow Leopard will make your MB faster for sure. You could use Activity Monitor and check how much RAM is allocated with your normal array of apps open... also check swap to see if too high (read about what should be using Google or this forum).
I would do Snow Leopard if it were me.
Good luck...