I've been a bit confused about this as well. I have burned several data ISO backup DVDs, using Nero 5.5.x.x (newest), and it shows both a yellow and red dashed line at the bottom, showing the "capacity limit" of the media, at "4500". And yet, when I fill it to nearly that mark, and pop it back in a DVD-ROM under Windows, it shows up as something like 4.23GB burned, and using DVDInfoPro, it shows as 4.23GB/4.5GB or something like that, meaning, that I've wasted a bunch of the blank space on the disk. I've been unable to adjust the position of the yellow/red limit bars in Nero, but am wondering "how high can I go" in terms of stacking data on a DVD ISO before I run out of room.
I guess that another way of saying that is, I'm going to go read up on the capacity limits, because I'm a bit confused over whether Nero, Windows, and DVDInfoPro are speaking in terms of GB or GiB, and what units the size limit is, as written on the DVD media packaging itself.
(Sigh. Why don't they just show capacity in "available LBA sectors", in all cases, maybe even in hexidecimal even. Would make life so much easier for us technical types.)