I believe you need to use a peecee to flash it. Then slip it into the Mac.
It will take a hell of a lot less time, and cause less grief if you just got the Mac Radeon 7000 card. You won't have to d*ck around with it to try and get it to work. Flashing the card could also void the warranty, depending on how you do it.
Again, I really do think that was only for nVidia cards, and NOT for ATI cards.
I would go with the Mac one, even if I had to wait a little while. Even though it is only about a $45 difference in price. How much is your time worth??? For me, that is about 30 minutes worth. I would rather be using it for those 30 minutes then trying to get it to work over the next several days. Top it off with the chance of not being able to return it. BEFORE you buy the OEM, ask the seller if it will work in a MAC... If they say no, then get the Mac one. If they say there is a way, get detailed instructions, and something from the store manager stating that they WILL take the card back if it doesn't work in your Mac. CYA