Exchange 2010 RC is feature complete and available for download. Download it from here
You can install the 2010 RC on Windows 2008 SP2 X64 and Windows 2008 R2 X64 editions. It cannot be installed on earlier versions of windows. In place upgrade of 2010 RC version to 2010 RTM is supported. The evaluation edition available for download is an enterprise edition and hence can be upgraded to 2010 Enterprise RTM (and not a standard RTM).
In place upgrade from 2010 Beta to RC is not supported.