Photoshop Questions

I made a design in photoshop and I am trying to move it into a templete. I forget how to keep all the layers together as I move it so I don’t have to move each seperate layer.

If I have a picture that is already done can I turn it into a 300 dpi afterwards? If so how do I do this?

I am using photoshop 7.0

Thanks for your help in advance.