I would never vote for the 6990s. The only dual GPU card I could ever recommend is the 5970. The 6990 and GTX 590 (although not part of the discussion) are plagued by the power issues, hence their downclocks, as the PCI-E + 8pin + 8pin are insufficient in powering the card to its full capabilities.
You'd be better off with 3 x 580s any day. They overclock way better, are not being held back due to the power, and nVidia scales much better in general. Just check out Anandtech or Tomshardware reviews on crossfireX and Sli scaling comparisons.
Also, I assume this is going to be for surround gaming (minimum 3 monitor setup) - as 2x 580s would be sufficient for 2560x1600 res gaming with everything max on nearly all current titles.
The only next big expected move is the Kepler generation GPUs from nVidia as Fermi is coming to the end of its lifespan.
If your options are the 3x 580s, you'd better be using at least 3 monitors at 1080p minimum, if not its a waste of money with excessive processing power. The 'sweet spot' is being capable to run your monitor(s) at 60fps average on max settings. Any gaming done at less than max settings only lets you experience a percentage of the price you pay for that game.
- my 2 cents