How to migrate from SQL Server 2005 to 2008
Solution 1:
It's easier. Run the install for SQL Server 2008 on your server, choose the upgrade option. Done.
Takes maybe 15 minutes, give or take. It really is remarkably easy.
Edit: You still want to do a backup anyway right before upgrading, just in case. But more than likely you won't need it.
Solution 2:
You'll also want to run the SQL Upgrade Advisor. This tool can help you determine if you're using any deprecated SQL features and will alert you to any gotcha's you might encounter.
Once upgraded, to take advantage of many of SQL 2008's new features and enhancements, you'll want to change the compatibility level of your databases from 90 to 100.