There is a great whitepaper the practices folks generated on MSDN.
If you want to understand the intricacies of upgrading a complex system to 2005, this should give you a really good idea what it's like and what to look out for.
http://msdn.microsoft.com/sql/default.aspx?pull=/library/en-us/dnsql90/html/SQL05InSrREAL.asp
Thanks,
Universe.Earth.Software.Microsoft.SQLServer.IS.KirkHaselden