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Too broad a question.

in some cases running two cards can be better than just 1
in others cases such as more power needed, more heat, the need for better airflow.
the usually official answer is, 1 better card is better than 2 lower end cards, everyone has differences on this, some cards work flawlessly for some, while the same cards offer nothing but headaches etc etc![]()
a 570 should handle most things quite nicely, what exactly would you be using the card for? Gaming? what resolution do you use?
You're better off with a single high end card.
No chance of SLI issues or micro-stuttering.
A single 570 will happily pump out good frame rates on a single 1080p display on all currently available and most future titles for a while to come
If the OP can afford a high end card then fair enough. Nothing beats crossfire price/framerate right now.
I've had no issued with my crossfire(2 x 6850) so far. But only tried about 4 games so far, all of them almost double the framerate. I think the only card that can come close to performance is the 6990 and that in itself is a very power hungry card.
)Is the AMD 6990 king at the moment?