FlightGear is a very versatile simulator. It has the ability of using multiple dynamic flight models (
http://wiki.flightgear.org/Flight_Dynamics_Model). It's not as polished as many commercial flight simulators (it's more polished better with every release), but it make that good by being an open platform to experiment with. And yes you can do some serious flying with it, that rivals the commercial flight simulators. I cannot give you comparisons with real aircraft since i'm not a real pilot. And on your computer it wil run perfect with alot of detail enabled.
I personally use the linux edition, because i like opensource and just works rock solid.
As a good starter resource goto:
http://wiki.flightgear.org/Portal:User.Also it's fun to do multiplayer with. You can watch your position, and that of others anywhere on the globe via a online map using google maps (
http://mpmap02.flightgear.org/).
Just go for it.
