This all sounds well and good, and I would crawl on my belly naked over broken glass to have one of these inside an Amione... however...
Well, a 600MHz cpu is what Haig at Matrox Tech Support have said will likely be the requirements for the card, so yes the G3 @ 600 should do just fine. But you are correct that it will not use it to it's fullest potential, because A1 currently only supports AGP2X. That's not to say it would be a total slow dog either. Just that it would not be able to reach it's maximum.
The Amiga uses Warp3D for 3D stuff, along with Mesa (OpenGL compatable). So it has a good foundation, and will be getting a few other things like input devices, and sound (?) etc in the upcoming versions. Someone else may know more than me about this, so feel free to jump in and correct me if I'm wrong.