play dvds through computer or xbox?  

Xbox DVD is 480i only. If you mod it and hex edit a file it can do 480p. All that being said, it is not a good DVD player.

The dash itself can be 480i or 480p however DVD playback will only be 480i regardless of dash version. MSFT does not enable 480p playback (you can hex edit the file after modding your box though - but this purpose is not enough reason to go through the learning curve as DVD playback is still subpar to any $100 standalone player).

Right, the dashboard yes. DVDs no. If you can't play 480i, you can't play Xbox DVDs.

Xbox as a DVD player is very bad.

If you're using a good enough PC anyway, you can download Zoom Player and FFDSHOW, upscale the resolution, sharpen it, and get an even better picture. Are you using VGA or DVI for your PC? Or something else? I use my HTPC for watching DVDs, and they look much, much better with upscaling and sharpening.

Check out the HTPC forum: http://www.avsforum.com/avs-vb/forum...?s=&forumid=26
This guide on using Zoom Player and ffdshow to improve DVD picture quality:
http://htpcnews.com/main.php?id=ffdshowdvd_1
(I suggest Zoom Player (have to pay $70 or so for Theater Tek), ffdshow, and WinDVD6 codecs)
HTPC DVD Playback settings thread: http://www.avsforum.com/avs-vb/showt...hreadid=447500
FFDSHOW FAQ Thread: http://www.avsforum.com/avs-vb/showt...hreadid=246752

I found that going back and watching the original Matrix (original transfer), it was very soft without my resize/sharpening. I also found that when I watched Chronicles of Riddick (Director's Cut), it was very sharp to begin with, but the same resize/sharpening settings brought out even more detail.

What I do is:
- Levels - adjust the gamma a bit to bring about black detail
- Resize to 2.5x DVD resolution with Lanscoz (parameter 2) and .6 Luma Sharpen
- Sharpen - Unsharp Mask @ strength 25

A lot of people use denoise, but I was never happy with the results. Mine is a basic resize + sharpen config.

I see a huge improvement using the method from the htpcnews article using Zoomplayer with ffdshow as cyberbri posted above. It approaches HD quality.

Be aware that your pc should have a fair amount of horsepower (at least a 2.6gHz CPU and at least 512MB RAM, plus a decent video card) but it blows away the results from the Xbox even attached through the hi-def pack.

You might be able to get a slight upscale with some sharpening. Don't worry about any denoise. Try just 2x DVD resolution on Lanscoz (parameter 2), and if that doesn't work, go down to 1.5x or so. If you can't do any resize, play around with the Sharpen filter some to see if you can get some more detail out of the picture (just don't over-sharpen).

One thing that I had to worry about was making sure my software plus Overlay renderer were outputting blacks and whites at the right level, in conjunction with having my display at the right output. I used www.displaycalibration.com and the Phillips Pattern Generator (http://www.mooneyass.com/testpatterns/) to make sure my TV's HTPC input was properly calibrated to my computer/video card's output. Then I used a test disc to make sure my DVDs were playing with the proper levels. I know that VMR might require some adjusting in FFDSHOW's Levels (Input settings). I boost the gamma on mine to 1.15, and boost the Levels Input to 5, to get more shadow detail and make sure my blacks are black after boosting gamma.

It can be pretty complicated. If you have the money ($200-300), an upscaling DVD player is another option, and does practically the same thing (no sharpening or user tweaks, though).