I'm doing something similar with Dirty Hot Productions (one of my clients). We use MySQL on a separate CMS Rails app to store everything, and all of the websites communicate with the CMS via an API. We use Redis to cache all of the API responses, so effectively everything except the first read (or the first "dirty" read) on a particular query is coming straight from Redis. It works amazingly well, and it doesn't use nearly the amount of RAM I thought it would.