I have a X1900, HD 2600 and 4 hard drives in my MP, which makes it slightly toasty. I've noticed I can hear the HD 2600 spinning up and down during heavy disk activity.
That being said, for normal users, it should rarely spin up. Worse case you can increase the Mac Pro's PCI fan to 750 RPM, which adds next to no additional noise. This should keep the card cool enough from ever spinning up. (I know it does on mine.)