The “retina display” has twice as many pixels as the old one:
iPhone 4S screen resolution. 640 x 960
iPhone 3GS screen. 320 x 480 pixels
And here is a comparison of the camera resolution
Click on the images to see full sized versions
camera image from iPhone 3GS 2046 x 1538 pixels
camera image from iPhone 4S 3264 x 2448 pixels. It's a little wider than the 3GS
