Note: SQL SERVER 2008 Upgrade Technical Reference Guide Download by Microsoft
This 490-page document covers the essential phases and steps to upgrade existing instances of SQL Server 2000 and 2005 to SQL Server 2008 by using best practices. These include preparation tasks, upgrade tasks, and post-upgrade tasks. It is intended to be a supplement to SQL Server 2008 Books Online.
A successful upgrade to SQL Server 2008 should be smooth and trouble-free. To achieve that smooth transition, you must devote plan sufficiently for the upgrade, and match the complexity of your database application. Otherwise, you risk costly and stressful errors and upgrade problems. Like all IT projects, planning for every contingency and then testing your plan gives you confidence that you will succeed. But if you ignore the planning process, you increase the chances of running into difficulties that can derail and delay your upgrade. This document covers the essential phases and steps involved in upgrading existing SQL Server 2000 and 2005 instances to SQL Server 2008 by using best practices. These include preparation tasks, upgrade tasks, and post-upgrade tasks.
Abstract courtesy : Microsoft
Reference : Pinal Dave (http://blog.SQLAuthority.com)