Copied from
ArchiCadWiki:
"The performance of your video card only matters when you navigate in the 3D window in OpenGL mode. The better the video card you have, the faster you can rotate around or fly through the model when working in the 3D window. The time it takes to generate a 3D model in the 3D window, a section/elevation, or making a rendering depends on the speed of your CPU and the amount of RAM available, not the video card.
So my guess is that pretty much any 2 year old GPU will be fine, but a pro-level card may have fewer display glitches.
If you have any further comments, I would love to hear them! Maybe ArchiCad 13 changes the rules of the game?