AS3 or AS2? A Compability question

I want to create a big project in flash with xml, php and mysql,
and i don’t know what to use: AS3 or AS2.
Im afraid that AS3 won’t be good for the target audience, who may have Flash Player 6 Max…
Sombody knows what is the common Flash Player nowadays?
I mean people do not suddunly install Flash Player…
And if i use AS3 to ad banner, maybe many people won’t see it properly.
Am i wrong?
Is AS3 useless?