What Is The Best Graphics Card I Can Get For A Mac Pro?

I have the Mac Pro (Desktop) and the standard graphics card isnt getting the job done. It’s a Radeon HD 5770. I was wondering if theres any more powerful graphics card thats compatable with the Mac Pro. I have the 4-Core 6gb RAM version.

Mac Pro best answer:

Answer by Angelo C
dont think you can be playing around with a mac, firstly make sure you are not using a mac os for gamming, secondly, the best thing you can put a 6990m is the fastest for any notebook