Format factory is the mutts n***s when it comes to video conversion, and the best thing is that it's free. Easy to use, reliable, supports every format i've ever thrown at it and utilises 4 cores aswell.
As for dvd authoring, i'd have to say ConvertXtoDVD as the all time best. Again, it utilises 4 cores and is as simple as pie, whilst giving professional options but without all the pissing around with too many sub menues etc.
It's not free but you can get a serial anywhere, if you don't want to pay for it that is.
You can use it without a serial, but you get an annoying print in one of the corners of the screen.
However the trial lets you see just how easy and versatile it is to use
A normal 1.5hr dvd will take my Q6600 at 3.4 about 8-12 mins to author it. All that is then needed is to burn it with whatever tool you like.
It does have it's own burning tool built in f you want to use it aswell