Cross, it depends on the computer you're using to decode the video, it will have to be decoded from divx/xvid to mpeg so it could take up to a day if not more depending on what you use to decode. The picture quality should stay relatively the same, if you convert it to vcd it will probably lose some, svcd not so much, straight to dvd probably none. I don't own a dvd burner so I have no clue to how that works. Oh and I used to use tmpgenc, I think that was the name, to do conversions between types. I haven't had to since my last format so I don't have a copy handy.

As for DVD to DivX I use dvdx, it's an all in one program but you have to run the movie for a bit before you use it, gives really good results though. The tut I read to get me started with it was http://rip.corusa.com/
There's a lot of useful info at http://www.dvdrhelp.com too