These are roughly the steps we've used to do the same thing.
1) Build the new server.
2) Backup your current databases to tape or other media.
3) Stop SQL Server.
4) Backup the database files (MDF,LDF and others) to another server.
5) Shut down the old server.
6) Bring up the new server with the correct name and IP address.
7) If not already installed, install SQL Server.
Make sure you create the same data directory structure.
8) Copy the MDF and LDF files to the correct data directories on new server. You should be able to do this for all databases including system databases.
9) Start SQL Server. Verify databses, agent, mail services, etc.
If any of the databases fail to load, restore them from backup.
Check here for an excellent article that has more detail than I've provided.
Terry L. Broadbent
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