Color and depth buffer

Hello everyone.

I'm new to unity, and I have some probleme to do something which should be (imo) simple.

I have a simple scene (some trees, water, skybox). And I have a button (GUI).

What I want to do is: when I click this button I'd like to save (or send) the color buffer (the image) and the depth buffer of my camera.

First question: How do I tell my project to be on rendered mode and not in forward ?

Second question: How to retrieve thos two buffers ?

sources: I heave looked at this and this but I'm still a little bit confused...

Script: Here is the begining of my script (which is not working):

private var cam:GameObject;
cam = GameObject.Find("Main Camera");   

function OnGUI () {
if(GUI.Button(Rect(20,40,100,40),"Tst")){

    PrintBuffer( _CameraDepthNormalsTexture);

    }

}

function PrintBuffer (r:RenderTexture) {
SendItSomewhere (r);// fct to save it or to send it
}

My english may not be perfect, so if I'm not clear don't be afraid to say it =).

Someone told me that once my texture is drawn, I should use it as a global shader property: _CameraDepthTexture ... but I can't see how to call this global, and how to use it to do what I want ?

Anyone have the slightest idea ? =)