I have a single-story 1400 sq/ft house (4 bedroom). Here's a VERY rough sketch of the layout:

The house has blown-in insulation on the exterior walls but no insulation on the interior walls. Windows are single-pane aluminum (we're getting ready to systematically replace these this year).
At the current time, we don't have any heating (the fireplace that's there is a standard masonry fireplace but we haven't used it (there's a gas line hookup nearby but it's not connected).
At this point, we're considering a couple options:
1) Get a gas-powered fireplace insert
2) Get central heating.
The fireplace salesman naturally said that his solution is the best (citing something about fireplace inserts offering better energy/heat transference efficiency than central heating).
I like the idea of a thermostat-controlled fireplace insert but I'm skeptical that something like this could heat an entire house.
So my questions are:
1) Is it remotely possible for a fireplace insert to heat an entire house? Especially considering the rough dimensions of my house?
2) Are Fireplaces truly more heat/energy efficient?