InsideHW.com has compared performances of single graphics cards vs. CrossFire/SLI systems
>> CrossFire vs. SLI
On every presentation of new GPU generation that we attended in past few years, systems with more than one graphics card were presented as ultimate performance setup. Problem with those configurations was their price in a real world environment. Even now there are more users that have only one graphics card in their rig but the times has changed and now even low-budget users can have Crossfire of SLI system thanks to price drops. First of all, motherboards that support Crossfire or SLI are not too expensive anymore as they were a year ago. Secondly, graphics cards have become also cheaper so users now face a dilemma: if they already have a graphics card from mid or high-range class with motherboard that supports CrossFire/SLI, is it better to sell that one and buy new one, or to buy same graphics card and make CrossFire/SLI system?.
>> CrossFire vs. SLI