Radius VPN and Active Directory

We have a Radius VPN server running on WinNT4 on an NT4 Domain and we are about to ugrade to an Win2003 domain.
The question is: What will we have to do to ensure that the migration will be as smooth as possible?
Is Radius will be compatible to Active Directory?
Will we be able to leave the Radius VPN server as is ?
Or will we have to reconfigure our VPN services ?

Thanks in advance for your help!