For dual monitors I think the G5 had that support in the stock card. If you want do use a program like Motion that utilizes the graphics card, then I would upgrade but if your using FCP for two monitors then stick with what you got.
powerdog,
I'm not an expert on videocards, but I'd suggest you to check on the specs of your
videocard and see what it has to offer. Like Geno said, I'd doubt you need an upgrade
for FCP. Good luck.
I run the same card and run dual monitors in FCP, AE, and every other program just fine. Every once in a while things get sketchy in Maya with the OpenGL, but I think that's more of a Maya thing. WIth FCP, it runs smooth as silk!