One of the few things that we know about the next release of Sharepoint is that it will run only on x64 platform.
So there are 2 Rules of Thumb:
- If you're deploying MOSS now, plan your infrastructure to be 100% x64! This is a must if you want to leverage the inplace migration to upgrade!
- If you're creating your "perfect" Sharepoint Development environment, with your favourite tools, settings, configuration files and all the stuff that usually requires a lof of days just setting up, well... it's a good idea to set them up on an x64 environment otherwise you will have to reinstall again!