Well, as I said above, Joudi, I have filled many DVDs with data, and I mean FILLED, right up to 4.7Gb, and they have all been readable afterwards. In fact, similarly to you, that's how I back up my files on my PC. I just copy the whole of My Documents to a DVD, and it usually just fits on. That's using Nero, with the same method as you described above.
Sorry I have no solution, but I can tell you that it does work. Have you tried a different brand of blank DVDs? I do know that some drives work better with some brands, and sone drives don't seem to like the cheaper brands.