Are you hosting dynamic content (PHP, Java, etc.) or is everything really heavily cached in static files/varnish and hosted out by nginx quickly so as to avoid saturating CPU or I/O?
There is also a wordpress site and another database-request-heavy site with 500 requests a day.
The problems I had was that the access.log and error.log quickly ate up the 5GB space of the micro instance :-) Causing mysql to stop operating and me to scratch my head for a few minutes.
5GB is not a lot .. you want ruby1.9 with the default setup? That will be 180MB, and so on :-)