Next step with web application single, java, recommend, different, learning

amy

While learning the basics of java web development, I put together a simple web app that is on a single server/jvm, uses Tomcat, and has standard web functionality - i.e. login/logout, basic security, etc.
Using Spring, how can I distribute functionality of (web and business) tiers over different servers?
Should I be looking more at RMI/JNDI or Web Services?
Can anyone recommend books, websites, etc. that can help me with the above two questions?

Thanks!