"Only Apple cards will work with OSX. If you want the X1900, buy it from Apple, and it will work with OSX and Widows."
This is very wise advice and will save a lot of headache. However, for the technical savvy, mavericks you can get a 512MB RAM ATI X1900 XT or XTX and flash it to work in both.
I can confirm this works:
http://www.tenthousandpercent.com/?section=other&article=76
Please do extensive research and make sure you aren't going to void any warranties with Apple.
And last, you absolutely need to buy this:
http://www.welovemacs.com/9227128.html
The card needs 150W at peak and the PCI-Express only supplies about half. From what I have read it's possible to go from the 2nd bay CD-ROM connector thing to the PCI Express card using an adapter, but the Apple aux power is highly preferred to supply adequate power. And also note even though the Apple aux power looks like a 4-pin molex, it is not. It's proprietary and requires the above cable to go from the board to the PCI Express card.