The SharePoint team blog has posted a great entry about the beta of the SharePoint capacity planning program as well as information about hardware recommendations. Basically you will need 64bit machines for the future versions of SharePoint. Not really a big surprise seeing how most of the other server software world is heading to 64bit. A few other tidbits there as well. Worth your time to check it out.
I have been using the capacity planning program for a while and I am quite impressed with it. Provides verification of hardware requirements that you present to customers.