First of all, you might want to take a look at the 2-site replication set up option, as this can allow you to tune a separate read-only public AtoM instance for high performance and availability. Essentially, you have 1 ready only public site and a separate internal staff edit site, with a replication script that can either be run on a schedule, or manually on demand, to duplicate the database and search index between the two. See:
You can also then set up an additional caching engine such as Varnish on the public facing front end and configure it to cache aggressively to increase response times, flushing the cache as part of your replication.
Artefactual uses such a set up for most of our Premium+ clients. I am not sure about specific allocations, but I know that for large installations, our team recommends provisioning a separate SQL server / VM, and then provisioning as much RAM as possible to both the Nginx/AtoM and SQL servers.
Hopefully other users might chime in with their own recommendations!