I haven't written node code before, so correct me if I'm wrong. The package.json file indicates MySQL as an optional [1] :

    "optionalDependencies": {
        "mysql": "2.0.0-alpha9"
    }, 

And this guide [2] seems to indicate that PG support is possible.

[1] https://github.com/TryGhost/Ghost/blob/master/package.json

[2] http://www.howtoinstallghost.com/how-to-install-ghost-on-her...