For most people there will be no significant benefit at this point. It's certainly faster at processing instructions, but the problem is many of the applications on the market haven't yet been re-developed to provide 64-bit instructions to the CPU. That being said, it's always very cool to run the latest, greatest, bleeding-edge code if you can handle the risk.
And here's the biggest risk right now: Not many of the more popular Anti-virus packages out there are capable of supporting 64-bit hardware. And a few of them simply will not run on 64-bit Operating Systems. Right now, the one I know of that's effective on 64-bit XP and Vista is Kaspersky. You can get a trial on betanews.com, and I would seriously recommend doing trials of all vista x64 products before buying them, as some will perform better than others.