The original source file, yes. Don't try to do that with the test file, because it's not good to recompress lossy sources over and over again.
There's several types of resizing filters. I used BilinearResize() in the previous examples because it's fast, but there are others that can give a sharper result. The documentation lists these others, but yes, you would use that filter to make sure the script is giving x264 a video stream that's 800x600, not 800x598.