It would be a fun experiment…how important are these 400 sessions to ya? There are so many questions here, I don’t think anyone can answer this without trying it first. Is there a peak time of day for these secessions? What kind of plugins are you using on WP? How complicated is your node/Mongo/App? At least you could add memory quickly here or better yet, isolate Mongo in a new dropplet and then do the same with MySql.

It generally peaks at 50 sessions per hour, but has once in the last month reached 70. The Node-Mongo piece just serves up some extra meta information (the posts are restaurant reviews, the Node bit provides the address, a rating, … to allow additional ways of searching).