Planning to upgrade to SQL Server 2008
If you are planning to upgrade your legacy systems running on SQL Server 2000/2005 to the latest version i.e. SQL Server 2008, start with this guide. It contains almost everything one needs to know for the upgrade.
SQL Server 2008 Upgrade Technical Reference Guide
Also One more must download is SQL Server 2008 Upgrade Advisor. It is a separate download. The most recent downloadable version is available as part of the Microsoft SQL Server 2008 Feature Pack.
If you are (still) running your systems on SQL Server 7.0, then upgrade to SQL Server 2000 or 2005 first and then to SQL Server 2008.