AFAIK, FCP doesn't benefit as much from the video card (compared to Premiere CS5, for example) because there is no GPU acceleration. All the rendering is processed by the CPU. you might see some benefit in Motion, however, in going with the 6750, if you use Motion a lot.
This may change with the new FCS update within the next few months - hopefully it will include GPU acceleration and (mother of god, finally please) 64-bit executable.