Okay, I originally bricked this card by flashing with a custom ROM that was corrupted by the EFI insertion process. I reflashed back to orig BIOS on a friend's PC so I could actually post with it in my Mac Pro. I figured out what went wrong with the EFI insertion process, fixed it, and reflashed. Now I have a fully working 4870 video card in windows and OS X, and the full GB of memory is recognized in both!
I'm currently running with the latest beta of OS X 10.5.7, and the 4870 is fully recognized. I know one poster had mentioned having trouble sleeping his system with this card, but it appears to work perfectly on mine.
The specific card I'm using is an XFX Radeon HD4870 XXX Edition 1GB PCI-Express Video Card purchased from a local Fry's.
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