For those wondering whether or not to use the 32 bit or the 64 bit version of Internet Explorer (if you use Internet Explorer), Computerworld has some good information on it. There is a good reason not to use the 64-bit version, but many times it won’t matter. I use 32 bit, myself, due to the plugin issue. I don’t notice any speed difference, so it may be noticeable, but it’s not a hindrance to any work being done.
They do this for a good reason: backward compatibility. Plug-ins have to be rewritten to work with the x64 version, and a lot of plugins have yet to be rewritten. More 64-bit plugins are being released all the time, of course, so this problem isn’t as severe as it used to be. For instance, you can now download and install 64-bit versions of Adobe Flash Player and Google Toolbar. But if Windows were to default to running the 64-bit browser tomorrow, it would still result in a lot of frustrated tech support calls.