How did you solve this? I have the same card.
well it wasn't too easy.
I followed these instructions:
http://web.me.com/jacobcroft/4870Flash/4870Flash.html
Not all of them tho.
Just the boot cd stuff
you see the problem I had was that you NEED the power cable connected to the x1900 to boot into windows (it'll run in os x without the power cable connected)
I re routed the 2nd molex connector from the CD-Rom bays through the bottom part and used the 4pin molex to 6pin adaptor that came with my video card.
I had the x1900 in the 2nd slot and connected directly to the main board, i had the 4870 in the 1st slot and connected to the re-routed power cable.
I made the cd as per the instructions and flashed the card using atiflash. Just make sure you flash the right card, (it'll be the one thats listed as 0 under the adaptor column)
After the flash, turn off the computer, remove the x1900 and plug both power connectors into the 4870 and reboot.
I could see the apple logo and boot into os x just fine. Seems to be working for me. And it's working fine in windows 7 too.
I have noticed some graphical bugs in os x sometimes, like on boot and animations arent as smooth (ie, opening and closing windows etc)