Update Update Update
You CAN flash ATI cards on a Mac, with no PC involved!!! Download this file for the flashing tool:
http://groths.se/atiflashingtool/atiflashingtool.zip
And this one for the ROM images:
http://groths.se/atiflashingtool/ROMS.zip
Search "atiflashingtool" on Macrumors.com for extensive discussion of various scenarios, as there doesn't seem to be much on the groths.se website.
Happy Mac'ing!
Dan