I am not completely current on the state of graphic cards, so I don't have any specific recommendations. Here are a couple of things to keep in mind, however.
Generally no graphics cards will be of much assistance with rendering in FCP7. With the exception of FXPlug plugins, most of Final Cut renders in software on the main processors.
Motion & Color do use the GPU, so a better card WILL help you there.
The Adobe package is also GPU aware, but you need a card that understands something called CUDA to really get the best performance from it.
Finally, be sure the card you pick is MAC compatible. OSX wants specific drivers. You may see the same model advertised for much less for PC's but it will have to be hacked to work on the MAC.