I optimized the sites on the 512MB droplet for a few weeks (mysqltuner, nginx configs, plugin selection) and then recently stepped up to the 1GB RAM. The admin pages were chugging a little, especially with commerce plugins in the mix.
* It made sense traffic/memory wise to go to 2gb, but all of the sites are 'behind' cloudflare and that seems to help a lot as the server rarely starts breathing hard...
In any case it is a small load, but we moved from a shared host setup where we had these old sites sitting and they were beginning to bring down the server. They weren't into installing APC or Memcached; we were able to handle the traffic on the 512mb server using Apache, Cloudflare, APC and Memcached and the sites are faster (also using Google's pagespeed module). I've upped the server memory now, mostly because the WP admin is slow and the cost is still much less than we were paying before...