32 bit can support up to 4GB of RAM although you are more than likely to only see and be able to use about 3.25GB of that.

An interesting thing to note however is that if you put 4GB of RAM in a 64 bit OS then you would see the full 4GB but since 64 bit by nature will use more of that RAM when running the same programs, it will perform in a very similar way to a 32 bit OS with the same amount of RAM. So, if you want to go 64 bit, I recommend that you only do so with 8GB of RAM or more.