If you're a gamer then a 64-Bit Vista will allow you to use more than 3GB of RAM properly, because a 32-Bit Vista (or XP) will only recognize up to that amount and not above (at least not by default). Again, if you're a gamer, it's the only difference.
EDIT: And perhaps something else to consider is the potential issues to make certain (only a few) programs to run, and also you'll need 64-Bit drivers for your hardware (64-Bit drivers for everything like sound card, graphics card, monitor, etc).