Your best bet with DVDD is to leave the settings at default, as once recommended by the author. The only exceptions I personally do are making sure the auto update is NOT ticked, as Macrovision now owns DVDD and is no doubt tracking all the hits for updates
, and I personally crank the buffer to max 256. By doing this I have noticed a complete stop to buffer "dancing", although not on the device buffer with select drives ie 2MB drives VS 8MB.
With ImgBurn, also use defaults, LUK knows what he's doing. I personally like changing the booktype to DVD-Rom for compatability, most people do. As far as speed, if your computer, drive and media are up to it, rated speeds are no problem. I have no problem's burning good media at 8X or even 16X, but I stick mainly to Taiyo Yuden & Verbatim media, with a small smattering of Ricohjpn to keep me honest