Not in all cases. 2 x 5770's outperforms a 5850 usually for example. Lots of times, you can spend the same amount of money on 2 less expensive cards and get better performance than a single card. Although, this of course is assuming that you already have a mobo/PSU that can support it.
AND in some *crazy* situations, I've seen over a 100% gain. In metro 2033 or Stalker CoP, 400 series cards.
For the most part, you see much more of a gain than 30-50%.
If you're talking new nVidia cards in SLI, you'll see at least 80%, they nearly double in most games.