Scalability

RDBMS: runs CPU-bound. Extra CPUs should be on the one machine.
Business logic (calculate state tax) can be kept in the RDBMS: faster access to data. If multiple RDBMS then maybe it can be in its own program.
Merging the result of the SQL queries with HTML is normally done in the webserver (tomcat, IIS+ASP.NET). These can be duplicated as needed.
HTTPS adds CPU overhead to your websevers: duplicate. Also, hardware encryption machines exist that are much more effecient.
10 dynamic pages per second is a universal constant.