The default scaling method for any lower resolution is the monitor switches to that resolution (display scaling). If you see black borders for 4:3 resolutions this is because the graphics card tries to keep the original aspect ratio. You can go at nVidia Control Panel and switch to stretch, so any lower resolution fills the screen. To improve sharpness you may also switch from display scaling to GPU scaling. It's all there at the display section. I don't currently have access to my PC to guide you exactly, but you get the idea.