First of all thanks a lot for your answer which are always very helpfull!!
Idid have some doubts about my last edit:
the originale images are 720x576 25/sec; At the "new project" I set HDV 720p and for export in DVD I set PAL_dolby_DVD and max quality; not according to the available space, so my standard video is made in a 4,7Gb DVD and lasts max 1 h.
Is that the best set I can do in order to get the best quality according to the original clip?
That's kind of a backwards way of doing things, since you're setting your project up for a higher resolution than your source footage and then rezzing it down to standard resolution to output it as a DVD.
Instead, start your project based on the specs of your original 720x576 video. Then, when you output a DVD from that, it should look as good as the original. (Depending, of course, on what model of camcorder the original footage came from.)