In terms of highest performance, yes it is worth it. A pair of GTX 560s or a pair of 6850s (some 6870s on sale too!) can all be had for less than $400 and will outperform GTX 580.
On the other hand, if you just buy a GTX 570 or 6970 now, maybe in a year you'll find a second one used for a drastically reduced price and get really crazy performance for a good deal. However, in the mean time you will have less performance so... yeah.
I'd really say SLI 560s or CF 6870s are the way to go for that price range.
I always think a single card solution is more promising, less problem, less heat, less power consumption, and less noise. Also, you always can add another one later in the future when the price drop...