The above way to set up your query is perfect.
....if you use the latest 5.7 build which is just about ready.
See:
http://developer.db4o.com/blogs/product_news/archive/2006/09/18/New-BTree-Field-Index-Processor.aspx
After you have had the chance to try out 5.7, please post back how the results changed. We would love to hear about it.