This is something I’ve yet to do, but I’m starting to look at this seriously now and I’ve found some useful resources for this.

Installing MOSS 2007 on Windows Server 2008 by Matt Hester (Technet Edge) – with a video walkthrough as well.

How to move MOSS 2007 from Windows Server 2003 to Windows Server 2008 64bit (plus they’ve got a load of other great FREE whitepapers to download)