Yeah man, at least according to
this guy, which seems he did some pretty extensive testing, I have to admit that was a pretty interesting read and it opened my eyes and im sure a lot of people too.
I guess people went crazy about it because first of all it gave them the "illusion" of having a better more expensive card for "free", but as you can see there the performance gain is minimal with shaders only, now add in the fact that if you match core and memory speeds there's been reports of cards dying, I don't know man, whether it's a 100% chance of happening or not I sure wouldn't want to risk it, you're playing with fire there.
EDIT: If you want my advice MisterAnderson I'd wait for more info to come out on the 6990 or the 590, if you can't then I'd advice you to go for the two 6950's the
1Gb model tho, cheaper and performs the same as the 2Gb one, with 2 of these babies you'll be way over the 580, your power supply seems to be up for the task too.