Unless there is some unusual skimping, you are getting the best you can from DVD.
As far as I know the TV is always displaying 1080 (1080i in terms of scan lines). Actually 960 (960i) for DVD is just as good as 1080 and in some ways better, but is rare and makes the TV more complex.
If the player is feeding in 480i, that is converted to 480p and (if the TV is not doing 960i on the screen) the 480p is converted to 540p.
If the player is feeding in 480p, the 480p is used as-is for 960i on the screen or converted to 540p for 1080i on the screen.
In terms of subject matter from DVD, the vertical resolution is still 480 no matter how good the equipment is.
Video hints:
http://members.aol.com/ajaynejr/video.htm