Minimum? Any single-core processor (Pentium IV is fine) and 1GB RAM. About 500MB of free disk is sufficient. That should handle an app with maybe a handful of EJBs, several dozen servlets/JSPs and about a dozen or so casual users. You might be able to get away with less RAM if you are running Linux (I would not try that with Windows Server.)
What technologies/libraries does your app use (EJBs, Web services, Messaging, JSF, Seam, Spring, other)? What is the size of your EAR or WAR file? How many simultaneous users do you imagine? Are you considering a 32-bit or 64-bit OS? Which OS? Which JVM version? JBoss AS 5.0.1 or 5.1.0? Have you done any performance or load testing? Do you know the heap memory requirements for your app when it is under load?