upscaling is just that, you can't take 420p & turn into 1080p or 720p. YES It is a night & day diffence between dvd's & BD's. IF you sit way past the max viewing distance of your sized tv then your not going to see much change, but you would still get richer colors & better sound.
There is NO technology that can turn a 420p disc (SDdvd) so it looks 1080p or even 720p.
420p is about 500,000 pixels (dvds)
720p is 1,100,000 pixels (TOA-HD, HD cable & HD satellite)
1080p is 2,200,000 pixels (BD's)
So if you can buy a DVD player that upscales to 1080i, how noticable could the difference on an Bluray player be?