SLI is indeed 2 Nvidia graphic cards which work together to improve your framerates. Crossfire is 2 ATI cards. You have to make sure your motherboard has the correct corresponding chipset, each brand has it's own set of specifications to run 2 cards. You can't run Crossfire on a motherboard with an Nvidia chipset.
SLI is beneficial when you run games at very high resolution. If you run at 1280x1024 or less, you won't see much benefit from 2 cards. Also, a single high-end fast card is most of the time better than 2 cards in SLI or Crossfire of a slower version. SLI won't result in double the performance just because you have 2 cards. It depends on the game, some are better optimized for 2 cards than others. Normally, you will see an average of about 30% improvement. Some games will do way better, and some games actually run the same or even slightly worse than on a single card. Running 2 cards in my opinion works like this, you buy the best graphic card you can possibly afford at the time, you have to bite the bullet and go for the top dog. After 6 months to a year, you may feel you card is just not quite doing it with some of the newer games coming out, and by that time your high-end card has dropped in price a lot. So you grab a second one and presto! New found performance.
The other scenerio is you just have a ton of money to spend, so you buy 2 of the best cards out there and put them in right away. Remember, buying 2 midrange cards and putting them in SLI will not be as good a single high end card, and costs more.