Don't.
Move any data from your 8Gb drive to your 40Gb drive that you can, just to free up some space and install all new programs onto your new drive, and use it to store data from now on.
Use a utility like Norton Ghost or Powerquest Drive Image to make an image of your 8Gb drive and store the image on your new drive, and burn it onto CD if it will fit. Drive Image is probably better as it allows compression of upto 50%.
The reason being that if you use the 40Gb drive as your boot drive, what are you going to do if it fails?
I have a 2GB boot drive and a large drive for everything else.