Now the one about clock speed is back from '05 so I'm going to assume that its wrong now. But I don't know. If anyone could tell me some legit, current info about what to go for then I would appreciate it. If proof, like performance charts, is given then I would love to see it.
I can't show you numbers or charts, but I do know that the newer AutoCad 3d versions have their own rendering machines. I believe it is called Accurender and the newest one is Mental Ray, which I know Revit has.
If you are rendering in AutoCad, go for more cores (quad cores work best in my experience) but if you're only drafting, you won't benefit much from a quad core. A Core 2 Duo with 2.5 should be plenty powerful.