So in a single post.
32bit allows you to install any older 16bit software you have. If you love breaking out games from the many moons ago, you'll need the 32bit copy. You RAM address space will be limited to 4GBs however.
64bit will allow you to use much more ram. 16GBs in the case of home premium, 192GBs for the other versions. At the moment few programs use more the 2 or 4.