I did the same with about 5 WP sites and another 5 custom sites running on DO at 512 (it's now on 2gb). One of the custom sites gets about 15,000 pageviews per day, and APC with memcache seem to handle WP on Apache fairly well for now. If you document your setup I would be interested; I tried to get nginx running on another droplet but lost patience. I'm sure I could have done it, but it was lower on the list ;)

* 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...