Okay, some progress...
I was able to extract the ubcd .iso and create a new .iso for CD that includes a folder with atiflash and the ROM, so now I'm able to boot that to DOS.
Now it's time to flash the card, but I do have a question... I should back up my original ROM (as mentioned by you, TR, and from tutorials)... but how?
I can boot the ubcd but don't have access to hard drive(s) (my attempt at creating a readable USB stick was a miserable failure). I could boot into Windows with the 4870 running and use GPU-Z, but I'm not sure how because if I have only the 4870 connected, I won't get the bootloader screen with the option for OSX or Windows, correct?
And I'm not sure if (through OSX) I can tell the system to boot into Windows upon startup. AND, if I can do that and the card flashing fails, I suppose I'll be able to put the X1900 back in and get the bootloader screen?
Thanks!
ETA: I see in system prefs - startup, that I can set Windows 7 to be the startup disk. I'll set that and load into W7 to back up my ROM.