Active Directory

Ok, I have a major concern (well, my boss does).  I have a Windows 2k DC that has about 50mb of space free.  The drives size is 8gb and the system sits on a 4gb partition (it's not my fault, I inherited the network when I was hired and now tasked to figure out this nightmare) and has the second 4gb partitions setup with Exchange 2k.  I have a second serer running Windows 2k3 that we want to move up to a DC as well, but are concerned that the AD on the 2k DC will not have enough space to update itself with the addition of the 2k3 server DC.

My boss insists that someone can give an idea of the minimum amount of space that will be needed, but I'm skeptical.  CAN anyone say how much space needs to be available on an existing DC when another is promoted to DC status for AD replication and other functions?

If we don't have enough space to update AD on the 2k server, I need to look at getting it on another harddrive with more space, but since it's an exhange server, can that be done without impacting mail in a longer that normal outtage to bring up the other machine?  I know it will need to have the same machine name and such.  That's a whole other problem.  But, I'm in a bad situation and have limited options with results I don't feel comfortable with.