Its very unlikely we will ever see a GTX590 as Fermi is just to far off the design spec. At best it will be a GTX480 style paper launch.
You really only have two Nvidia options, a pair of GTX480's (be warned they are poor cards - read the GTX480 thread for details) or a single GTX580.
I don't think a 590 based on 2x 512 chips is realistic at all, however if one does come out (as it's been noted to be in some beta drivers, though I wouldn't be surprised if they did it on purpose to "prove" the 590's existence), I reckon it'll be more likely 2 560 GPUs.
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