> Should I use HttpSessions or statefuls Session Beans, or both?
You'll almost certainly need HttpSessions, just to keep the context. A List or Vector of JavaBeans (NOT EJBs!) is usually adequate for a shopping cart - each bean would typically contain a product code (SKU) and quantity or something like that. Save the EJBs for the catalog lookup.
> Should I use a XML Framework for presentation, e.g. coocon2?
I think XML for presentation is a bit overhyped, myself. I prefer to save it for data transport. It's extra overhead when you're just converting the XML to html anyway.
> What job can XML do for me as interface between the tiers?
In this particular case, no benefit I can see.
> Can I use containermanaged persistence even if I have SQL statements that result in sums an aggregations?
Not easily, since the sum is a one-way item and unless you have read-only EJBs (which aren't really explicitly supported in
EJB 1.x), you'll have a great deal of difficulty explaining to the container how to persist the result field.