What Video Cards work with the newest model of the mac pro?

I am looking for someone that can give me a clear answer on what video cards work with the newest mac pro. i know that the Radeon HD 5870 works with it because you can choose it in one of the options on the website. but i am looking for something better than that. like a 6990 or something as powerful, quiet, and that doesn't get SUPER hot. the 6990 is what i want but please tell me what cards there are that i can get right out of the box and is compatible with the mac pro or the ones that i need to flash. I also know that the 6870 i don't need to flash but it isn't has good as the 5870.
Please tell me what i am saying wrong and tell me clearly what graphics cards you know about that work. This is getting Really frustrating!!! I have tryed many other places to see if anyone had the same question but no luck.

 

Thanks.
Helped a lot