Does the Exchange Server Beta support Windows Server 2003 x64 Edition?

yes, it does. The full list of system requirements are at <http://www.microsoft.com/technet/prodtechnol/exchange/E2k7Help/1e80857c-b870-4a6d-a0f4-ff7b3a7be037.mspx?mfr=true>. You'll see that Windows 2003 x64 is the only supported OS; there will be an unsupported 32-bit version that you can use for demos and so on, but for production use it's x64 or nothing.