i was just researching that very same question in light of the upcoming AC15.
on the mentioned spec page i find the recommendation regarding video cards for macs somewhat generic...
Video card drivers for Macs
Since Macs come with a default video card (with one optional choice at some series), and the drivers are bundled with system updates, we can not recommend a specific card or driver. Mac OSX's graphic engine, Quartz does not allow an application to directly access video card functions, which minimizes the possibility of application-specific display problems. On Mac there are no third-party drivers, and the drivers Apple builds into the operating system are very thoroughly tested for OpenGL compatibility, since the Quartz graphic engine itself is based on OpenGL. This is very different from the PC platform, where you have hundreds of different video cards available, drivers from different sources, most of them optimized for games and DirectX, not for CAD and OpenGL. You can be confident that all of the current Macs come with a video card that is compatible with ArchiCAD.
unfortunately, the link to the AC15 video card recommendations (http://archicadwiki.com/VideoCardsForArchiCAD15
) does currently not allow access... hopefully they fix this soon.
in our experience however the above is true at least for the current imac range that ship with AMD Radeon graphic cards - though from memory there was an issue with AC10/11 with some graphics cards displaying fractal patterns in the 3d window (sorry can not find the corresponding threads atm).
if it is of any interest, we usually get the top of the range imac (currently 27inch, 3.4GHz, 4GB RAM) and upgrade later with additional OWC RAM modules (at least in AUS this is much cheaper)... but it really depends on what type of work you typically do.
btw. you should your system spec to your signature.