One of our clients is upgrading their servers because the old machines can’t handle the load of the database anymore. They have been using sql 2000 for the last 6 years and the db has grown to hold a few GB of data.
Will it be worth upgrading to 2005 or 2008? What are the major benefits of the new versions compared to 2000?
I you are planing to upgrade from SqlServer 2000 I would skip 2005 and go directly to SqlServer 2008
It has all the features of 2005 plus some extras (for example an option to pass a table variable as a parameter to stored procedure, new date types, spatial data handling,etc.)
You can refer to Advantages of MS SQL Server 2008 over MS SQL Server 2005 question for the comprehensive list of features
EDIT
I can see that the question has been updated and now SqlServer 2008 is included in the question.