Sounds for 2006 Pontiac g6 sedan?

Without knowing the wiring of your head unit I'll attempt to give you some good suggestions.

To answer your first question, yes there are bass option outside of a trunk installed subwoofer. These are small subs that go under a seat and some have an amp already built in. The best can be found at crutchfield.com

As for your second question, in order to amplify you would need an amp and they wary widely in price depending on brand and power. If you just run 4-6 stock speakers off it then you won't need a high power amp.

The only real issue is the head unit. It may or may not have the wiring connections you'll need to install an amp. Some aftermarket head units can perform the same functions as the stock units but I'm honestly not sure which ones will or will not for your model car.

The best way to plan this out would be to go to crutchfield.com and they have an "outfit my car" link which will show you what will and will not fit/play well with your car.

I would say if your gonna custimize and add power to it I would look at the cheaper cars like the cobalt or the avenger.Make sure if you buy either of them to get the 2.4 motor.I am not sure if it still does but a neon with the 2.4 was the record holder for a true unibody front drive car and if I remember right it did in the eights in the quarter mile.So their is a lot of performance potential with the motor.The reason I am recomending 4 cylinder cars is because you haven't listed any v8's the chrysler 5.7 hemi with mds gets pretty good fuel mileage for a v8 and can compare to some v6's for fuel mileage.V6 engines are flawed from the factory as they are never truly able to be balanced.In a v6 2 pistons are up on one side while only one is up on the other.Manufacturers compensate for this thru harmonic dampening.It doesn't eliminate the imbalance but dampens the vibrations.That is why early v6's were notorious for vibrating.Good luck on your choice.