for the cable box set it to 720p or 1080i since your tv can not do 1080p there is no need to change it for a box that can do 1080p.
1080p would only be good on that sided tv if you sat 3 feet or less, so no big deal that it is 720p. 1080p is best on a tv 50" or larger

for the new dvd player, you just have to goto the menu settings (dvd player) & change the resolution to 720p.