Dear God. Search, search, search, you... eh. Do your own freaking research.
EDIT:
In the spirit of Christmas, here's something less hostile.
The x264codec can be used to encode both HD and SD video.
Most HD videos are better than their DVD counterparts, unless you use some sort of horrible encoding profile that gives you half a screen of squares. I'm not even sure if that's possible.
Regular DVDs use, I believe, the 480iresolution (or 480p, I dunno). Anyway, even 720p is, compared to 480p, much higher in quality. With a regular DVD title, you have... about 400,000 pixels. With a 720p file, you have over 900,000 pixels. That's more than double the pixels. (With a 1080p resolution, you've got well over 2,000,000 pixels, which is more than 5x the DVD resolution).
So yes, x264 can encode both HD and SD movies, and the quality it produces from HD sources is almost always better than DVD, and the quality it produces from SD sources is almost always nearly identical to DVD.
What you have to realize, though, is that encoding, any sort of encoding, is going to result in a quality loss. If you're re-encoding a DVD video into something like DivX and Xvid and even x264, there's going to be some amount of quality loss. The only way to not lose any quality is to keep the DVD movie or transcode it into lossless .avi, which takes up a LOT of space.
