Consider this:
Do you have a computer with a 64 bit processor? if not you probably have a 32 bit system.
It is logical to me to use the respective OS to the type of System you have i.e. a 64 bit OS in a 64 bit system.
In short:
If you have a 32 bit system (CPU is 32 bit) use Vista 32, if you have a 64 bit system (64 bit CPU) use the 64 bit Vista.

You should know that a fully operational 64 bit system needs 4GB of RAM (anything less and you are not using it to its full capacity).