Networks

First install the OS but don't add all of the SBS features.
Next run a DCPROMO on the new server and migrate the FSMO roles to that box.
Now it will depend on what else is on the old box. If it's just a DC and file/print server you can proceed. If you have any other special programs that you are planning on migrating, now is the time to do it.
If you have nothing else there of consequence you can complete the SBS install procedure.

If you ahve any problems getting it to finish run a DC promo on the old server so that it won't be a secondary domain controller. You shouldn't have to do that though.

Best of,luck