I can't advise on the Mac Pro, although I must admit that I was seriously considering purchasing one but decided against since the current version is rather 'old' for the price. Instead I have the new Retina MacBook Pro......and it is fabulous !
It runs ArchiCAD flawlessly, and also e-on's Vue Infinite which is (in my opinion) considerably more demanding than ArchiCAD. From what I gather on this forum, most users of the iMac are happy with their performance. But, as you say, a sufficient amount of RAM is important. Before the RMBP I was running on 3MB RAM and it was insufficient......now that I have 16MB it is great. I would have thought that a current iMac with atleast 8MB RAM, and preferably a solid state drive (SSD) would crunch through ArchiCAD with ease...
AC26 Intel & Apple Silicon (preview). Twinmotion.
M1 Max MacBook Pro, MacOS12