If you are a TechNet Plus Direct subscriber, you'll see the non time bombed RTM bits available in the subscriber download center. I can't tell yet if the .ISO image includes both the x86 and x64 versions. I'll update this post after my download completes.
If you aren't a subscriber, then head on over to http://www.microsoft.com/technet/prodtechnol/eval/exchange/default.mspx and get the evaluation versions of the product. The eval area has both the x86 and x64 versions. Remember, x64 is the supported production version of the product.
Before you download and install, you might want to check out the tips and tricks located at http://www.microsoft.com/technet/prodtechnol/exchange/default.mspx. There are a number of good resources there. And of course, don't forget to review the Exchange Server 2007 product documents at http://www.microsoft.com/technet/prodtechnol/exchange/e2k7help/.