Do the ones you've backed up play OK in your XBox?
I assume your Xbox is modded in some way & that you are backing up using the ftp route.
Chef is correct about the burn speeds & media IMO.
Some others disagree with what he & I would say & do support the slow for quality route. All I can say is that with quality media then rated speed works very well for me.
ImgBurn (development of DVD Decrypter) would be the best choice as the burning app, I wouldn't attempt this with Nero.