Obloquy, Mithadan was talking about Sauron practicing necromancy on dead Elves, not dead Men. Nothing can bring back a Man who has died, except Eru Himself.
We know that the Nazgul were Men, who had been enslaved by the Nine Rings. These Rings were originally made by and meant to be used by the Elves of Eregion to slow down the fading of the Elves, but Sauron had "assisted" the Elven-smiths and introduced some of his own "improvements" to the Rings. These "improvements" were there to make it possible for the One Ring to control the other Rings, but they can't have been too obvious or the Elves would have noticed Sauron's treachery earlier. Therefore, the Rings weren't inherently "evil" and they must have had some possible "good" uses as well (just like the Three).
These good uses were probably only accessable to Elves, as Mortals would fade themselves if they wore a Ring. We know that if a Mortal's life is influenced by a Great Ring, he'll become "stretched". We have three examples of stretched Mortals: Gollum, Bilbo and the Nazgul. The latter ones had faded, while the former ones had not. I believe that this is explained by the Hobbits' resistance to fading (Frodo proved this by carrying the tip of Morgul-knife for so long without fading).
Gollum and the Nazgul had become immortal because of their Rings. Physical wielding of their Rings was no longer necessary for them to continue their existence - after they had worn their Rings long enough, it was enough for the Rings just to exist. Bilbo was another case, as he hadn't apparently worn the One Ring enough to become immortalized. It made him long-lived though.
On to the Nazgul. They were originally mighty Men, and Sauron gave them the Nine Rings he had stolen from the Elves. The Rings made the nine Men great kings, warriors and sorcerers, but their lives had become unending misery. Their bodies (hroar) faded into the Unseen world and their spirits (fear) became attached to their Rings - this means that they never died to become Wraiths, and that their hroar and fear never separated before they became Wraiths. I believe that their hroar were necessary for them to be in any contact with others but not necessary for the Nazgul to continue its "life". If a Nazgul's faded hroa was destroyed, I think that Sauron could make him a new one (just like a dead Elf may get a new body in Valinor). This is supported by the fact that they were crushed in the flood at the Fords of Bruinen, but they were able to return (even though it took time). I also seem to recall that they were taken out in the War of the Last Alliance as well, but I can't find it anywhere in the books right now.
So what was a Nazgul's hroa like? I'd say it was probably just an unseen version of the Man's old body. We know that they had unseen sinews, thighs, heads, hair, etc. so Tolkien can't have been using the word "Wraith" in the traditional sense (at least my dictionary defines "wraiths" as immaterial). They were also limited by their hroar - they were never able to pass through things, like a traditional ghost.
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