I did a swing migration, 2003 to 2003, you need 3 boxes, the old server, a temp server and then the new server.
You create the temp server, DC promo it into AD , transfer the 5 FSMO roles accross to it and make it the Global Catalog, dcpromo the old server out.
Then setup the new server, and then dc promo it into AD, and then transfer the FSMO roles to the new server, and make it the GC.
Then dc promo the temp server out and boom, all your ad, securty, etc etc is all functioning on the new system.
TIP : you can use a VM for the temp server, if you dont have a 3rd box.