Currently there aren;t really any cards that actually add hardware capabilities to Final Cut Pro. Some older cards did this by adding specific FX to a daughtercard, but Apple stopped supporting that with version 4 and the RT engine. RT extreme is a Mac Preocessor based function and the amount of real time you get is purely dependant on the speed of your Mac computer and actually, the speed of your drives. For example: if you have an xserve raid you can expect far more RT, than if you have a firewire drive.
When you launch FCP, you might notice that there is a portion that says (profiling for RT extreme) that is FCP determining what It can do with your RAM, CPU and Drives.
NEWS FLASH. The cards are only the through port, they add nothing to the RT capabilities of your system. The card is just the Piepline, that’s it.
David