Depends on how many tables you have but there are several things to do before re-arranging your models including caching, caching, caching, query optimization, code optimization, minimize database access, move static assets to CDN, etc.
MySQL or Postgres are fine. In general, better to grow into that size of a site, not to pre-maturely optimize on Day 1 (that's not to say build an unscalable site on purpose; just don't paralyze yourself). --
