Tomcat Minimum Memory :: Virtual Hosts vs. Multiple Instance

Trying to determine memory usage in a vanilla web app run through tomcat.

I assume that a virtual hosts setup will use significantly less memory than host-per-instance. What is the minimum memory footprint of a single host tomcat 7 instance? For each instance added does the memory footprint grow linearly, or can we share common resources am开发者_开发知识库ong instances?

I would prefer a multi-instance setup, so as to isolate client sites (i.e. not affect other sites on redeploy or restart), but memory usage is the key. If each instance requires 512mb ram (like grails, for example), then I may have to take the virtual host route as I was not intending to use the 16GB ram available on tomcat alone!

Suggestions appreciated. BTW, only a handful of sites will incur significant load; the majority are small and draw on a client CMS (perhaps I can virtual hosts these sites and only host-per-instance with the "important" client sites)