The 50/1.4 on a 20D is going to work just fine but keep in mind it is not the same thing as an 85/1.2 on a 5D/1Ds.
But if you have to ask this question you probably shouldn't bother buying the 85/1.2. Buy the 85/1.8, 100/2, 135/2, 100/2.8 Macro, etc..
I've had the 50/1.8, 50/1.4, 85/1.8, etc.. on a 10D, and 50/1.4, 85/1.8, and now 100/2.8 Macro on the 5D.
The 50mm lens is going to give you a lot more DoF no matter what camera you use it on, it's not going to look terribly like any of the others.
But all the other combos whether with a 1.6x or FF body are going to give you the ability to blow the background way out of focus.
Only if you are a specialist in this area of next-to-zero DoF portraits and unhappy with the other lenses would it be worth it to go get an 85/1.2. The 85/1.8 has more then enough DoF control on either type of camera to get focused eyes and an out of focus nose tip & ears. The 1.8 also is of course inexpensive, small, has blazing fast A/F, and has excellent image quality. You can't do any harm trying the 85/1.8 first before going for the f/1.2 lens.
The bigger question is whether you want a field of view of 85/100/135. Everyone has a different preference there, that is much harder to figure out without trying all the lenses yourself, perhaps over an extended period of time.