You still have not said what games and other applications are important to you. Its just computer 101 that you start with the uses and applications when evaluating hardware. Surprised how many forget, even after being reminded.
Also what is your spending strategy?
1. Do you want the least expensive model card that gives you good current performance in your important games and applications? Note that any speed over 40 to 50 FPS will not be noticeable nor impact game play.
2. Or do you want to also add a little room for future proofing?
3. Or since you have a fast system, do you just want the max power you can get for the $400.
Further, your board allows 2-way and 3-way SLI support. Do you want to be able to do either? To use it now if it gets the best price/performance card? Be able to use it later to upgrade by simply adding another card? I like to preserve the last option - but it means using an nVidia card - as you seem to know but everyone else seemed to ignore.
I prefer nVidia cards anyway, which has an added advantage if you use PhysX or CUDA to assist the CPU in processing. As far as nVidia card brands, I prefer BFG because of OC and service - but they recently announced they are exiting the graphics card business. Coincidentally the second best is EVGA and there certainly could be some benefit from having a graphics card and mobo from the same manufacturing - although I would not pay a lot more than other first run manufacturers and would take one of their sale bargains if available.