I installed it in my Mac Pro 3.1 and it worked wonderfully! But it's true, you have to work with the Nvidia Webdriver!
I also recommend keeping a second card, preferably the original Mac Pro one.
As soon as Mac Os X or the web driver are updated, the graphics card is sometimes not recognised, so you have to access it blindly (remotely) or start via an old card and install the previous driver.
With a Mac Os X update, the web driver is often uninstalled, which means you have to blindly reinstall the web driver again!