If the monitor has a DVI connector the scaling can be controlled from the GPU drivers. You can set a fixed aspect ratio scaling (no distortion, but black bars), full screen scaling (the usual way), no scaling (the picture uses only the center pixels (black stuff all over) and the scaling can be done at the software end so the resolution is scaled before it goes to the monitor (the results are much much better).
I don't know about the situation with ATI GPUs, but Nvidia supports this, except the support was broken a while go (97.xx drivers IIRC) and I don't know if it has been fixed in the newer drivers.