I'm using an amazon ec2 1 core /1gb ram instance with Openemr on a text based Ubuntu server (14.04) installation and is more than enough for 20 simultaneous users. (2 providers).
For storage purposes the /var/www is mounted on amazon EFS using NFS with the direct connect method by modifying the FSTAB file.
Please note that I'm not using the optional couchdb database for storing modules.
Hope this helps.