Raytrace 3D is a deprecated feature and will simply be gone one of those days, so quite frankly I wouldn't spend a second thought on it. Any money you can spend on hardware can be better spend on 3D plug-ins or buying and learning a "real" 3D program. That said, support for multiple cards is entirely a driver thing, but given the lack of sophistication and performance in the Raytrace 3D renderer it will probably not do much.
Specifically, I have the NVIDIA Quadro 4000 in mind.
The NVIDIA Quadro 4000 card been removed from the recommended GPU list for Premiere Pro CC 2015. If you are thinking of building a new edit and motion graphics computer system, I wouldn't recommend building it with an older GPU. I would also pay attention to what Mylenium said about ray-traced 3D workflows being obsolete.
I guess I didn't realize that the ray-traced 3D renderer was obsolete; I was used to it, and it's only been around since CS6. After you told me that, I looked into it and discovered the new Cinema 4D After Effects integration. And thanks for the tip about the graphics card; I'll look into a different one.