Thanks for your comments.
As I wrote in my first post, I'll do an AVC coded file at 720p - initially at 25fps, then at 23.976fps, then an AVCHD disc. I also wrote that I'll do a PAL DVD...
I know that upscaling is a bad idea, but because AVCHD @ 720p (or 1080p) is the only format I could make a progressive version at 23.976, it is just necessary. But I (roughly) calculated that using five AC3 soundtracks @448kbps, the video at 1280x720 (really 1280x544) could be compressed at around 7200kbps, quite good for the AVC codec.
Eventually, I could also release a file-only version at 576/23.976 (not standard, but no resize necessary).
I used a simple ResampleHQ as Anthony suggested, and the result is not that bad. But could be better; I'll use the best resize method you all will suggest. I also thought to resize the four captures with different resize methods, to have "slightly different" starting points for median & average; I don't know if this could be useful or it is better to upsize the final clip.
About Youtube links, that's right, it's offline... they send me immediately an advice "... it seems you use some FOX material blah blah blah..." so I preferred to delete the video to avoid any problem; here you are the clip on Sendspace