renderpasses and rendertargets

hello is it possible to render the whole scene to a rendertarget ( a texture) and use that for post processing in another pass? if so, how?

also how can you easily set up a screen-aligned quad and a camera that looks exactly at that quad?

Yes, using Unity Pro. They're called RenderTextures, and yes, RenderTextures are used for post-processed image effects already. Unfortunately, this feature isn't available in the free version of Unity. It's very easy to implement in Unity Pro; just drag an image effect onto the camera, tweak a few variables, and you're set.

I'm not sure what you mean by screen-aligned quad. Do you mean a plane? Because setting up a camera that looks exactly at a plane is quite easy; it's just a matter of adjusting the camera's rotation and position for a moment to get them correctly lined up.