Exchange Server 2007 hardware requirements
Exchange Server 2007 requires a server with a 64-bit processor and a 64-bit operating system. It supports a variety of 64-bit processors, including Intel Xeon or Pentium processors that support Intel Extended Memory 64 technology (EM64T). 64-bit AMD Opteron and Athlon processors are also supported. Exchange 2007 will not run on the Intel Itanium (IA64) processor though.
I'm not sure if these other hardware requirements are official yet or not, but the information that I got from Microsoft was that Exchange Server 2007 will require a minimum of 1 GB of RAM (2 GB recommended) and a minimum of 1.2 GB of hard disk space.
While I am on the subject of hard drives, I should mention that all partitions must be formatted as NTFS. This includes the system partition and partitions containing Exchange Server binaries, transaction logs, database files, and any other Exchange-related files.
Exchange Server 2007 operating system requirements
If you are planning on purchasing a copy of Exchange Server 2007 when it is released, plan on running it on a 64-bit version of Windows Server 2003. Presumably, Exchange Server 2007 will eventually run on top of 64-bit versions of Longhorn Server whenever, but obviously that won't be an option initially.
Exchange Server 2007 will be on the market long before Longhorn Server, so Windows Server 2003 will be the only operating system that will support Exchange 2007 for now. In a way, that's a good thing. It means that you won't have to struggle with learning a new version of Windows while you are trying to figure out Exchange 2007.