I just received the X1900XT, and noticed on the box that it is only supported by Windows. Now, I know it can be used on the Mac - based on what Apple recommends and reading about users saying it is so great on their Mac.

Is there any thing special I need to do when I install the card? I will probably wait until I get some reponses before I try to install it, so I don't mess anything up...

This might help:

http://ati.de/products/radeonx1900/RadeonX1900G5/index.html

It says:
"Due to a printing error, some packages of this product are incorrectly identified as Microsoft Windows Vista™ ready"

However this product looks like G5 product....And in the Mac section of ATI I don't see the X1900XT Mac version listed.

I am afraid you just purchased a PC version of the card. I think the Mac version of the model is not yet out in retail.