why would you install active directory or DNS on the new 2003 server? It’s exchange, it needs to only do exchange. I wouldn’t recommend installing AD/DNS with exchange in the same OS space.
If you have limited hardware options install a virtualization software on the new Enterprise 2003 box (or upgrade to 2008 w/Hyper-V) and setup an instance of windows 2003 standard to run your AD/DNS functions. Then another instance for your exchange 2007.
I am not going to re-invest the wheel here, the instructions to setup exchange 2007 in your domain:
Tried and test.