I've been using my 720p for awhile now. I don't think the difference will be so extreme as to go and buy another expensive TV. My roommate has a smaller screen, but his is 1080p and honestly, I can see a difference but it's nothing so extraordinary I feel the need to upgrade.
It's like the difference between a Blu-Ray and a DVD playing in an "up converter" DVD player.
I can't say for certain, but I think the One will be that way also. As long as it's HD we should be good!