You get the benefit of a large framebuffer in an SLI\Crossfire which helps when you're running at extremely high resolutions such as 1600x1200 and beyond. IMO, dual-GPU systems are good only if you have a large monitor and want to game at aforementioned resolutions. Otherwise, the extra power consumption and heat output would have you spend as much as (or even more than) a single, more powerful card on good cooling and a good PSU. What's more, dual-GPU setups don't even offer 2x the performance of a single card, so the performance benefit is not what you'd (theoretically) expect it to be.