You might be confusing the output resolution at which the signal is fed to the PJ with the resolution of the picture.
You get best results if the output signal from the computer matches the native resolution of the PJ. This way you don't use PJ's internal scaler which is inferior to the GPU.
However if you use ffdshow you might find that resizing the picture to double the DVD original resultion (or some other resolution) might yield better results. This however gets scaled to the output resolution by your graphic card. For the output resolution see above.
You get best results if the output signal from the computer matches the native resolution of the PJ. This way you don't use PJ's internal scaler which is inferior to the GPU.
However if you use ffdshow you might find that resizing the picture to double the DVD original resultion (or some other resolution) might yield better results. This however gets scaled to the output resolution by your graphic card. For the output resolution see above.