I don't have a copy of 7 so I can't speak to that, but I do have 6 and 8. I can confirm that 6 has NDOS (4DOS), but 8 has some utilities that 6 doesn't such as DISKREET disk encryption, Norton Disk Doctor and Speedisk have hdd disk compression compatibility build-in, and single-pass DUPDISK (or it may have been called DISKCOPY) disk copy program. And as krebizfan said, 8 was also the first version to include Windows 3.1x utilities such as Norton Disk Doctor for Windows, Speedisk for Windows, INI Tuner, INI Editor, and a few others (going by memory here).
If you're looking for more specifics, I have NU 6 loaded on my 386 and NU 8 on my 486. I can provide more details if you need.