SLI stands for Scan Line Interleaving.
The only reason I went out and bought the second cards, is Far Cry 2 is coming out soon, and it looks awesome. I really want to run it well. That, and I shouldn't have to upgrade for a while now, which is nice to not have to do. It's true, SLi does use quite a bit of power, but I'm running my full rig with a Thermaltake Toughpower 650, so you don't need to buy a kilowatt PSU like so many assume. As far as heat, get a well ventilated case and up your fans speeds on the cards manually and you'll be fine, I mean your box will still keep you warm in winter, but it's not putting out puffs on smoke. Okay, enough of my SLi rant. Hope this helped.