Recommend some hardware for me

[H@rry]

I have a Nikon D40 and I want something to allow me to connect it to a computer and see the image on the screen before I shoot it to help me with the focus (and other things). I don't know whether there's something I can attach to it or whether I'll need another camera. If I have to get another camera I want to stay with Nikon so I can use my glass.