Overspeed is there to let you know that your breeching the maximum speed at which the aircraft can travel safely, and by travelling overspeed you will run the risk of severely damaging the aircraft. Otherwise known as Vne (Velocity not exceeded)

Different aircraft have different limitations, and to me 340-360 knots is about maximum that most airliners will fly. You have to remeber that this speed is your Indicated airspeed and not your True (ground speed), your ground speed is the speed you are travelling across the ground and your indicated airspeed tells you how fast you are travelling through the air that you are in, and because at higher altitudes the air is thinner it wil cause the gauges to 'under-read' the actuall speed of the aircraft. The gauges will 'under-read' your speed because when an aircraft travels at low altitude the pitot tubes which supply the gauges are being bombarded by more air particles which will give a more accurate read out, whereas higher up, the air gets thinner so the pitot tubes are bombarded by less air particles yet you will be travelling at the same speed. Thus giving you an incorrect speed.

So if you look on your GPS and find your ground speed it should be around 500kts (if you are travelling at 360 kts Indicated)

Few!  

If you are intending to get a pilot's licence one day learn to stay within the a/c flight envelope.
It's fun to do things in the sim that one would should NEVER do in real life!!!!!
Remember that most a/c break up soon after VNE and you wouldn't be able to write to the forum "Why am I dead!!!!"
I highly reccomend you take one or two trial lessons then you will understand the difference between flying in the sim and real life.
Vololiberista