Due to plenty of new features, upgrade to SQL Server 2005 from SQL Server 2000 is a very good choice. In case of SQL Server 2008 it provides noteworthy enhancements mainly for BI and enterprise architechture.If you are not holding BI implementation like SSRS, SSAS and also you do not have a number of SQL Servers connected in various domains configured in your organisational structure then hardly you will feel any shortcomings in SQL Server 2005.
I have upgraded to SQL Server 2005 and found it very satisfactory. Later i may look for SQL Server 2008 when i have to implement SSRS.