I don't think of it as 'moving UP' from 35mm to medium format. I think of it as moving sideways. A medium format camera is not better than a 35mm. It's different.
35mm cameras allow you to shoot a lot of hand held and spontaneous shots. With your F5 you can easily grasp the moment, even if you operate it manually. It's a great camera, and don't get rid of it when you get into medium format, because you may regret it.
Medium format cameras are often used on a tripod, they slow you down somewhat, and that can be both good and bad.
There are three solutions for getting into medium format:
1. Rangefinder. There are Bronica, Fuji, Mamiya, and older folding cameras. They are fairly light, they handle much more like your Nikon than a Hasselblad or Contax medium format camera. Most of these cameras handle very nicely, and can be hand held and used spontaneously with great success!
2. Twin-Lens reflex. These have one viewing lens and one taking lens. These can also be hand held with great success, especially since the shutter mechanism is so smooth and nice. It's also a good way to shoot 'inconspicuously', because few people expect you to look down into a camera to make a shot, they expect you to look 'through' it.
3. SLR. This is the Hasselblads, the Mamiya RB/RZ67 and 645, Pentax 645/67, Contax 645, etc - and in my opinion the ones that most often require tripod mounting, mainly because they're large and somewhat bulky. With that said, I love hand holding my Hasselblad. It works really well after you get used to it. But it's nowhere near as fast as a Nikon F5.
All three categories are fantastic solutions. I've used Fuji range finders and love them. A friend has a Mamiya 6, and that's just an awesome camera. I have owned and used Yashica and Rolleiflex TLRs, and I loved those dearly, but regretted not being able to switch lenses (the Mamiya C330 has interchangeable lenses, by the way, and it's a TLR). Then I've also used, and still use, SLRs. I've had a Mamiya 645 which was an excellent camera with kick-a$$ lenses, but I love the square, so I use a Hasselblad almost exclusively these days.
You have a lot to think about. All I can say is choose wisely. I've had many cameras that were great, but few that completely suit my way of working. The Hasselblad is the ultimate camera for me, it may be something different for you. I like it, because it's the same format as my Holga, Agfa folder, and ZeroImage pinhole cameras - and that helps a great deal when I make prints.
Good luck.