Seeking to upgrade video card. Any recommendations?

Hi everyone!

I use a 2006 Mac Pro, with a Dual Core 2.66Ghz Intel Xeon Processor, 8Gb DDR2 RAM, 4 internal HDD (surmounting 3 Tb in total), and an NVIDIA GeForce 7300 GT graphics card, with Snow Leopard (10.6.8). I use it mostly for editing and Compositing with Final Cut Studio 2009 (FCP 7 and Motion 4). Up until now, it has performed perfectly. However, recently, I've updated most of my plugins and effects, and some of them I cannot use any more, since it says FCP cannot render them using the current graphics card. I'm at a crossroads here: Is it better to buy a new, faster computer? Or should I just keep upgrading it until the technology goes obsolete? If so, what graphics card would you recommend? Are there any other components worth changing?

I post this question here, and not on the main Desktop Computer forums, because I believe it's a very specific problem for editors.