Help: DVI/ DVD, Nad t-534, Sony 3100 ES, Marantz,

The only others worth considering imo are 1.k or more...and not found used very often like The Onkyo SP1000.
The Denon 3910 for some reason does not show the Macro blocking as much ...or at all in 720p and should be an inmprovement over the 2900.
The Denon 5910 offers the very best video...bar none.
The Onkyo sp1000 has better audio than the 5910 and almost as good of video and is what I will be using until Blu Ray gets here....but these are 1500.00-2500.00+ street.

As far as your list...
For video the 59avi....for audio the Marantz.
There is the new Pioneer 79avi that supposed to have improved audio for less than 1k street that might compete with Marantz for audio.
The Pioneer has a great rep for quality control if buying used also.

I can't comment on the NAD or Cambridge as I have seen neither and I am not sure exactly how picky you are with video/audio quality.....but I highly doubt either of these would be an improvement for video over the 2900.

If the 2910 did not offer much improvement....the Pioneer 59,79avi,Denon3910/5910 and Onkyo SP1000 surely will....and the Sony 3100 es may also.

Thanks I will try to get info on more mid-range dvd players and then take the plunge on a top shelf one once the HD stuff (blu-ray etc) pans out in the next year or so. I have heard (not confirmed) that cambridge audio is basically an Oppo under the hood so to speak..
cheers

Just be aware that the Oppo has not only Macro Blocking problems on a lot of displays....but also some lip sync issues.

Is the 5910 really that good of a picture? I am seconds away from ordering one but just cannot quite do it, it's expensive.

Order one from Crutcfield with a 30 day no hassle return.
I would only buy the Denon if I wanted the absolute best video from one box available.
The Onkyo SP1000 is a better audio player, same build quality and just behind a bit on video.....maybe 5%.
The main thing the 5910 does is make not so great looking dvd transfers look better as compared to these other players.....maybe 10%.
On very good to great dvd transfers were talking maybe a 5% subjective difference between the 5910,SP1000,79avi.

A Pioneer 79avi using 480i via hdmi to a good scaler such as an Iscan ect will probably be better yet for just video on great transfers for less than the 5910 list cost.

It all ends up depending on your budget....as well as whether you want the best video only...using a seperate player for audio....or if you want the best compromise of the two in one player for under 3.5k.....or whatever your budget is.

I use a pj with over 100 inch screens so the differences in these players become more evident imo...and on screens under lets say 70 inches these differences may lose some value.
I compared a highly rated Panasonic s-97 to my current Onkyo SP1000 and could see a subjective 20-25% difference in background noise and detail...as well as color saturation.
Comparing for yourself in home, under calibrated conditions is always the best way to go imo...if you want the best for your set up.
Tje 5910 is the current best one box solution for video...but only you can answer whether its worth its cost over the other players.