You are right - a whole bunch of things can effect your tuning. You didn't say whether your load was predominantly with the EJB or the servlet side of things. Are you micro-tuning your EJB numbers as well with jboss.xml, etc? Stopping thread and connection churn here may also be of benefit.
Anyway, if you view this forum thread http://www.jboss.org/modules/bb/index.html?module=bb&op=viewtopic&t= I quote some reports that give some idea of factors on the servlet side of things. JVM and servlet container count as well in the scheme of things. In the webserver test, they throw 350Mb of heap at the environment.
The main thing on the servlet container is the number of persistent connections as you want enough threads to service them, and correspondingly more heap space.
Generally, for an e-commerce server, you have plenty of memory so it is not unheard of to start at 128Mb or 256Mb and work upward. But you would do tuning to find the sweet spot in a commercial environment. For run of the mill, dedicated servers start at 64Mb.
Hope that helps - it's a bit vague and rubbery.
Thanks, thats helpful,
we are planning to launch a basic Jboss hosting package in a couple of weeks an I am trying to figure out how much heap space I should allocate to each Jboss install. Our servers are have a 2Gigs of RAM and I need to find out how many jboss instances can safley squeaze on to each box.