Buy a monitor which can maintain the aspect ratio of the input image. Otherwise the 16:9 input of the PS3 will be vertically distorted. AFAIK most Dell monitors don't have any scaling options and will always stretch the image to fullscreen and distort it.
I've seen scaling options on some Viewsonic monitors. You can either display the image at the native resolution (will be a box in the middle of the screen the same size as the actual input). Or it can stretch the image until it horizontally exactly touches the edge of the monitor (It will have black bars on top and bottom) so you have no distortion.