It has since been discovered that the Region 2/4 PAL disc of Jedi is slightly more detailed than the Region 1 NTSC. So the best would be:
ANH - Region 1 NTSC
ESB - Region 1 NTSC
ROTJ - Region 2/4 PAL

But I must stress that the quality differences are very, very small.

As for your suggestion, resizing up, filtering, then resizing back down to target res is a common trick - some filters work better if they have more pixels to play with. Upscaling with one of the new EDI filters may give better results.