From Ken Stone:
Graphics Cards for Final Cut Pro and Motion
Conclusion
Most editors will be perfectly suited with the NVIDIA 7300 GT. It is a fast high quality graphics card that suits the needs of the average video editor. The ATI Radeon X1900 XT is faster then the 7300 GT and makes the workflow with advanced 3D much more efficient. Note: A faster and more expensive graphics card is not going to give Final Cut Pro and Motion users any additional functions or features. The 'only' visible difference lies in the speed. But as everybody knows that time is money, a faster graphics card would definitely be a good investment.
With the upcoming release of OSX 10.5 and new application updates, more 3D effects will become possible, requiring more and more graphics power from our Macs. With only an additional $249 for the ATI X1900 XT to pay, this graphics card should be the first choice for the professional video editor.