I think that the Xinside policy of not giving you an utility to tweak your video modes ( like xvidtune ) and/or to import your existing XFree ones in the evaluation ( and AFAIK commercial ) version is incomprehensible. I've spent about three hours putting this together ( hint: I've compared the VESA 1024x768@70Hz entry in the two formats ( and I'm nearly an electronic engineer ;-)) while an Xinside programmer could have written a comparable article in a fraction of this time...
I haven't downloaded any evaluation version from 1.3 onwards and I really hope they have fixed this. Well, if they have, this mini-HOWTO could be considered useless but, alas, if you read it you will learn something more about how everything works...