ok, from the digital point of view, just because its digital doesnt mean it doesnt get processed, it just isnt being converted to analogue. the DVD player still processes the info being sent out of the DVI/HDMI port to a degree, hence the fact some players offer options to alter the black level etc...
so its possible to get a better picture from a more expensive player, however this probably wont be so noticable unless you are using a very large screen, then it may play a difference....
obviously for analogue outputs the more expensive a player usually the better the picture, but again its more obvious on a better display.....
for me the biggest difference is in the sound though, the more expensive the player the better the sound quality.....and thats what i actually get more of a kick out of, i think somewhere its listed that around 80% or so of your brainpower is dedicated to what you see when watching a film, but i get so much pleasure out of sound i reckon my brain is tuned more on a 50/50 level..lol
the only other thing is that technology filters down, todays £150 players are using stuff from 2 years ago's £600 players in a lot of cases.......
the only thing i would avoid is a player thats too cheap (under £40), they break down regularly, often have a greenish tint to the image.....useful to buy if your a student or on very low income, not useful if you are a hifi/av nut...heh
If you will have the display to do HD resolution then wait for a HD player, if your display doesnt do HD resolution, then grab a DVD player now, demo a few at different prices and see what you think yourself...