Final Cut has a 4GB limit, so anything else is a waste of money. You can allocate 2.5GB to the program and it keeps 1.5 GB for whatever else it wants. However, it never tends to use that much. You shouldn't have other intensive apps open while editing anyway.
An external drive (preferably RAID 0 and Firewire 800) is your most essential upgrade. Never save media to your system drive. In that case an SSD is fine as a boot/program drive. But SSDs do not work well with large files, like the ones created for DVDs -- but again these could and should go to the external drive.
A powerful processor helps with realtime editing of effects and the like if you use a lot of those. I understand the graphics card matters only for Motion and Color, but I don't use those programs.