OK these are the specs (all in mm):
Width (Nut/24th), Radius, Thickness (1st/12th)
Ultra --- 42/56, 430, 19.5/21.5
Jem ---- 43/56, 430, 19/21
Wizard - 43/56, 430, 17/19
Wizard II 43/56, 400, 19/21
the Ultra is supposedly very close to the original Jem, but the Jem got increasingly more Wizard like. (From the DBK specs on this site: "kind of a Wizard neck at the nut but not at the 12th fret"
You also have to take into account the general shape, luckily for us the names correspond to the shape, so:
Ultra, Viper, Wizard = U-shape, V-shape, W-shape.
What most companies refer to as D-shaped is U-shaped in Ibanez terminology.
The W has a flat bottom, the V a pointy bottom, the U is round.
The Ultra is only a hair thicker than the Wizard II, but will feel differently. I recently tried an RGT3020 and I found the neck way to thick and round for my liking, but according to Rich the neck on the RGT320 is a lot flatter. For comparison, I own a Jem777DY (orig.Jem shape) and a Jem90HAM (Wizard-like)