I've had an HDTV for over a year now, but just never got around to buying a progressive scan DVD player until now. Will I notice a jump in quality over 480i?
Only if its a model with a top-rated de-interlacer (Faroudja or Silicon Image chip) - the cheaper models probably won't de-interlacer any better than your HDTV does. I'd recommend a Panasonic RP82,XP30,XP50 or Denon DVD-1600, DVD-910, DVD-2200, DVD-2900.
Depends on how good the line doubler is in your set and how good the de-interlacing is in the dvd player.
If that set is a Pioneer, Sony, or Toshiba you won't see as big an improvement as you would with most Mitsubishis, and if the player is a $50 loss leader no-name you won't see as much if any improvement as you would with a decent major brand player.
The advantage of the Faroudja and Silicon image players is mainly seen when playing video based material or poorly flagged dvds, won't necessarily show on a well-authored film based disc. I've got a Faroudja chipped Panny and a current model JVC and with non-problem discs actually prefer the JVC. With video based stuff and poorly authored discs like Red Violin, the Panny really shines.
My set is a 2 1/2 year old Sony KP57HW40. I have the Panny, which outputs 480p via component and 480i via S-video connected via both component and S-video thru my receiver whose onscreen menus won't show via component. This allows me to use the onscreen receiver menus while doing audio calibration with AVIA. I sometimes switch over to a regular dvd and forget to switch inputs on the from S to Component. The pq is better on progressive component but the difference is not so apparent that I don't sometimes forget to switch back over to progressive component inputs on the tv.