One congrats on the N/F80!!! And yes you can use the SB-50DX with the pop up flash at the same time as the main flash. It works very well I used it all the time when I had the combo that now seems light years ago. I use the flash with the diffuser over the pop up flash and the main flash in the bounce position it was the best of both worlds. I used to know the combined GN number but time, age and my brain have all data dumped that info.
Again congrats on a great camera. I also had the battery pack MB-16 for it but be warned it is just a battery pack no additional functions or portrait release. But it sure does make holding the N/F80 a lot easier. B&H has a used one for $29 bucks.