There’s a post here that’s very good about load optimization and performance:
Steps to Optimize WordPress in Regard to Server Load?
It might be a good idea to also utilize a CDN for a majority of your page requests.
If you want performance you’ll have to minimize requests to your database and setup aggressive caching.
Working with Drupal i know this can be built into your drupal install. I’m not very familiar with WordPress and if there are capabilities integrated into WordPress to facilitate reverse proxy requests.
If you’re going to use mem-cache you may need more Ram to serve up the page.
You may want to also setup varnish on your server. Setting up varnish alone will give you a big boost; as i’ve been told.
Varnish http://www.varnish-cache.org/