how much better is having SLI than having just one of the same card? i read it's not double the performance.
Anywhere from 0% gains to 80% gains. Depends on the exact game you play, whether it works with SLI or not, and whether your nVidia drivers are tweaked for that game very well. May also depend on the actual graphics card you're using.
I don't think you can really achieve a 100% gain (which would be double). So it's still best to invest in one single graphics card, getting the best you can afford at the time. The whole SLI deal is meant to allow for cheaper upgrade paths for the future (adding a 2nd card when it's cheaper instead of replacing with an expensive card).
My guess at random would be an average of 30-50% FPS gains across the board?
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.