The reason I asked about fixed aspect-ratio scaling is because it's the best solution to running fullscreen apps designed for a 4:3 aspect ratio on a widescreen display, assuming you don't mind the black bars on the screen edges.
Unfortunately, a lot of widescreen displays are stupid and won't do this sort of scaling; they'll just blindly stretch the image to cover the whole screen. However, your graphics driver may have an option to do this scaling itself; check the display section of its control panel. For example, I know NVIDIA drivers have this ability, since I use it with my laptop's 1280x800 display. The driver simply uses the GPU's scaling hardware to do this, so there's little or no performance cost.