I've found SLI doesn't work as well at low FPS. If you start regularly dipping below 30fps, the microstuttering can be a real nuiscance. On the other hand if the fps stays above 30fps most of the time, it becomes a real dream. So if you have a game that's just below an acceptable FPS with a single GPU, the second GPU will make it silky smooth. SLI also lets you crank up the detail & AA, which is always nice.