Therefore if l had an accident in a 1999 peugot 206, what speed would l have to be doing to inflate the airbag on inpact.

l think it is 20 mph

you bought a Peugeot. It probably wont work.
or
did you buy a peugot? what make is that?

it inflates at 600mph, goes off with a bang.

It is not about speed. It is about deceleration (how quickly you stop).

Single car impact for most air bags is 35 to 40 MPH. car to car it is about 25 to 30.

Car airbags are not designed to go off at a certain speed. They go off when a certain amount of G-force is forced onto the car in an accident.

carswoody has the right answer

Just before the car reaches zero mph following extremely rapid deceleration !

All cars with airbags if you have a crash go off at 30mph or above. 1 of my friends had a crash and he was doing 20mph and it didnt go off then and a person tried to say he was speeding but the airbag proved that he wasnt.

It is not the speed which sets of the airbag, it is the rate of deceleration. Hit something hard (a wall?) above generally 19mph will set it of. Hit something soft at the same speed (the rear of a car) and it probably wo not fire as it is not needed.