You can achieve millions of hits a day on a 512MB droplet, if you follow WordPress best-practices. Start with How To Install Wordpress, Nginx, PHP, and Varnish on Ubuntu 12.04
. You'll also want to add memcache, APC, and a CDN for static files. A caching plugin like W3 Total Cache also does wonders.
Think twice about using Multisite, and consider going with How To Set Up Multiple WordPress Sites on a Single Ubuntu VPS
, instead (but substituting Nginx for Apache). There's a newer article, also, but I haven't gotten a chance to go through it: How To Configure Single and Multiple WordPress Site Settings with Nginx
I tried Multisite for a while and was frustrated that not all plugins support Multisite (including -- at the time, at least -- many of the popular ones). While other popular plugins did (technically) support Multisite, you could quickly tell that they were designed for single-WordPress installs and were merely subsequently stretched to "support" Multisite -- w/o revisiting fundamental framework decisions; in other words, many plugins that "supported" Multisite did not work as well as they did on single-WordPress installs.
Not sure about edublogs.org, but WordPress.com utilizes Nginx loadbalancing.
Varnish is an HTTP accelerator and a useful tool for speeding up a server, especially during a times when there is high traffic to a site. It works by redirecting visitors to static pages whenever possible and only drawing on the server itself if there is a need for an active process. This tutorial covers installing wordpress on a LEMP stack (with nginx instead of apache), and then installing varnish.