Depends on your needs, I'd think: Servlet 2.5 and a JEE 5 complaint container should suffice for now, but note that you will need even a servlet 3.0 compliant container in case you want to make use of JSF tag libraries that rely on servlet 3.0 specific features (like PrimeFaces which comes along with [i]web-fragment[/i] entries).
And some JSF 2.0 features (like the passing of method parameters in the Unified Expression Language) will require a JEE 6 compliant container (JBoss 6 or Glassfish 3 at this point of time).
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