The image does not HAVE to be stretched. You have a choice of stretching it (proportional or non-proportional) or not stretching it at all. If you do not stretch at all, it will look every bit as good as on a 19" LCD, with the same performance. However with games, proportional stretching of the image doesn't look too bad.
Assuming your using the DVI connection, and any recent NVidia video card driver, your video card driver will give you the choice of:
Display Adapter Scaling (stretch to whole screen)
Centered Output (No scaling whatsoever)
Monitor Scaling (stretch to whole screen)
Fixed Aspect ratio Scaling (Proportional stretch)
Fixed Aspect Ratio is recommended, but you can use centered output is you don't want the image stretched at all.