1. If you are using separate hardware, promote your 64-bit OS to a DC and configure DNS. You would have to update the new DNS server IP address in DHCP and all static IP machines unless use the same IP address on your new server.
If you are using the same hardware, you have to demote the DC re-format and install a fresh copy of the 64-bit OS.
2. There is no need to move DHCP if it is on a separate Server.
3. It is not recommended to install Exchange on a domain controller. You need to prep the forest and domain which should be automatically done during the Exchange 2007 install. Once Exchange 2007 is installed and part of the existing environment, migrate mailboxes and public folders off of the Exchange 2003 server.