I would like to answer my own question.
I decided to carry out a scientific experiment.
1. I took several photos with my camera set to 16:9 aspect ratio and high resolution.
2. I downloaded a trial version of DVD PixPlay.
3. I created a test slideshow with DVD Pix Play using a number of images I had previously changed to a pixel height of 1080 (which I thought would be appropriate for my high definition LCD TV) as well as the new images which I left at their original (higher) pixel height.
4. I burned the test slideshow to a DVD.
Result:
1. I found that there was no difference in quality between the two samples.
2. The quality (resolution) of the images in the DVD Pix Play slideshow was much higher than the test slideshow I had burned to DVD using Photoshop Elements 5.
Conclusions:
1. I had wasted my time changing the pixel height of more than a hundred images to 1080.
2. DVD Pix Play produces a much more acceptable DVD slideshow than Photoshop Elements 5.
Finally, I am really engaged in two projects at the same time. The first is to create slideshows and the second to archive the same images to a high quality DVD. This means that I want the resolution to be 300 to 500dpi for the archive. Strangely I find that, whilst I scan the images at 500dpi, when I open them in Photoshop Elements 5 and select resize they are shown as being 72dpi. I have posed this anomaly on the official Nero Forum website, though I don't think very many people visit the site.