Your Mac Pro should already be a fairly capable editing system. What kind of HD are you planning to edit?
Upgrading the graphics card will make no difference in editing and rendering speed in Final Cut Pro. Applications whose performance is directly influenced by graphics hardware include Motion and Color.
You’d be better off increasing your RAM to 8GB – the best configuration for your model of Mac Pro would be 4x2GB, with 2 slots filled in each riser.
Final Cut Pro’s editing performance (overall responsiveness and number of concurrent realtime streams) is directly proportional to storage throughput speed. If you can’t afford a true hardware RAID, you could always create a striped RAID-0 volume using three internal drives in your Mac Pro, leaving the fourth drive dedicated to the OS and applications. Just make sure to have regular backups of your media, as RAID-0 offers no redundancy. Never store video on your system drive.
What formats of HD are you planning to work with? Codecs like HDV are fairly processor-intensive for editing, and rendering speed in FCP is mainly limited by CPU speed. If you want to speed up rendering time in FCP, buy a new Mac Pro with the fastest CPUs you can afford, and make sure you have adequate RAM.