What they seem to be trying to say is @1920x1080 or above, your cpu is fine for any of those cards.
That having been said, single cards over $300 are a poor investment, imo. They don't get you much more than the bragging rights of having the card. a SLI or xfire solution of cards a couple steps down from the 580, etc. will give you better performance for the same price.
Even if you motherboard doesn't support SLI/xfire, I'd still recommend a single card in the $250 range at most (7850, or 480 if physx is worth the power consumption to you), and save the extra money or use it for something else.
The 3x6770's ($330 total) I use perform on par with a 7970 in most cases. The only noticeable differences come with massive texture loads that need more than 1gb RAM, but that happens very rarely at 1920x1080. I turn off xfire whenever I'm not gaming to save on power.