Its not a cut and dry as the wording of your post leads me to think you would belive. (sorry thats a mouthful).
SD screens, like plasmas, have a lower resolution PAL DVDs (in a sense).
A PAL dvd has a resolution of 720x576 maximum
An NTSC dvd has a resolution of 720x480 maximum
The thing is though, that the 576 in the PAL dvd for the number of vertical lines is interlaced, they are converted to progressive by either the dvd player\scaler or output device.
If you've only got an SD output device then an upscaling DVD player is not going to be giving you all it can for your money, pretty pointless.
If you've got a HD screen, then an upscaling dvd plauer can scale the picture to a much closer match to your screens resolution. So really its just (the intention anyway) a better scaler than the one in your output device.
But it is designed for the source that you are using, the one in the output device would just be a general purpose one, an upscaling dvd player will be specifically designed for things like scaling the picture in a video whilst eliminating noise\artifacts at the same time and not accentuating them, as possibly a built in plasma\lcd scaler might.
Different plasmas\lcds have different quality scalers so its impossible to say for certain that any upscaling dvd player will improve the picture on any plasma\lcd (I'm talking about HD screens here).
(There's also the de-interlacer on the dvd player to take into account, but this isn't strictly to do with the upscaling, and I'm tired)
