I've got a 60" lcd rp 720p...

and an older dvd player with s-video output connected to it. Would there be a readily noticeable quality difference if I bough a lower end dvd player with hdmi output?

There should be a noticeable improvement between S-video and component or HDMI.

Composite and S-video are not ideal. Yes, component and HDMI would be a dramatic improvement. You spent a lot of money on your display. If you watch a lot of movies, treat yourself to a decent DVD player.

Good things are not cheap, and cheap things are not good,so you get what you pay for.