I work in the HT department in my local bestbuy (I am not the ignorant salesman trying to push useless shit down your throat)
Anyways, I am assuming your plasma is much bigger than your CRT therfore the "upscaling" should help abit to compensate displaying on a larger screen. Dont forget that your limited by the DVD it's self which is recorded in 480p.
What will give you the best image quality on your plasma is a good upscaling dvd player, not some shitty ass one like a Prima. It's really the chip inside the DVD upscaling DVD player which will make the image quality difference.
I personaly recomend the Samsung 1080p model DVD-HD960 , even though your plasma doesnt support 1080p, the engine in it does quiete a good job at upscaling.
On top of that the Samsung 1080p model comes with a free HDMI cable, which is pretty cool.