*** First let me say that I do not recommend testing of this sort in production. ***
Instead, you should look at setting this up in a lab. If you have hardware constraints, excellent options include Microsoft Virtual Server/PC and VMware Server/Workstation.
There are two types of scenarios: transitions and migrations. In this case, migrating would include installing a new Active Directory forest and Exchange 2007 organization (there can only be one Exchange organization per Active Directory forest) and migrating users and mail data and then decommissioning the E2K3 servers. Transitioning on the other hand consists of installing Exchange 2007 into the existing Exchange 2003 organization, moving users and mail data and then decommissioning the E2K3 servers.
Good luck and remember to have fun!