I haven't decided to put SQL 2008 on yet. If I do, I have the Standard edition. Express still limits you to 1 CPU and 1 GB of RAM, so I don't know how much of a performance increase you will get. I am certainly not a SQL Server expert, and my guess is 2008 would have advantages over 2005, but it still has limited RAM.
Have you thought about increasing the amount of RAM on your server in general?