It may or may not be rude but I agree with Tesselator to 1st use the search engine.
1. How long will it take for someone to post the "correct" answer?
2. The thread discussing "the fastest Mac Pro GPU's" may or may not cover a card that is best for Cinema 4D
3. The "fastest" card pertaining to what?" Some cards are fast for games and others for 3D rendering etc.
4. Use search and type in Cinema 4D for more specific "finds".
5. Using "search" 1st can possibly save you weeks of waiting on a competent response.
6. Go to the Maxon site and if there is a forum look to see what cards "they" use.
MacPro 2008 3.1 | 14GB Ram | Quadro 4000 | MC6.0 | OS 10.6.8 | MOTU HDX-SDI | ATTO R644| LaCie 324