Is there any reason to use 64bit MySQL (and OS) on small databases?

You already answered the question yourself.

If you are 100% percent positive not to grow beyond 2GB of RAM usage in the current hardware lifecycle you can stay with 32bit, when the time comes to replace your hardware you will have to reconsider anyway.

Given the current hardware market I don't see a real reason not to use 64 bit it will be the safe side in case of unforseen growth, that is true for cache as well as real data size. After all a full migration will take somewhat longer than just taking the box down and adding some RAM.

Drawbacks could be that pointers on a 64bit system use about 30% more memory but that won't really hurt you if you really stay on the small side of memory usage, but on the other hand if you grow you will be able to grow more quickly (at least as long as you can stay on the same hardware box)

64bit. If for some reason you get a request that blows the database needs beyond the 512, you can just add some cheap ram and be done with it. No reinstall, etc.

Running Mysql 5 or later on a 64bit OS is usually worth it regardless of the memory you're addressing because of the internal use of 64 integers in the database. Unless you're trying to fit your install into a tiny amount of RAM there is almost no reason to choose a 32bit install of Mysql.