I have FCP 5.1.4 also, and I find it's gated not by the video card, rather, by the speed of the system, and to a lesser degree, amount of memory. When I encode, I turn off FCP displaying what it's doing, so that everything can go into the encoding.
To maximize it's encoding speed, ensure you have lots of empty disk space, have maxed out the memory, and of course nothing else running while it's encoding. I tilt the back up 3/4" or so, to get the maximum amount of airflow for cooling. Continguous disk space can be a good thing for video usage, and this Apple note talks to it:
http://support.apple.com/kb/HT1375?viewlocale=en_US . Too bad they stopped making 7200 parallel ATA hard drives, as that could help encoding speed a bit too.