I personally (as well as many others) recommend a single very powerful card over 2 less powerful cards in SLI/Xfire.
Always choose faster single card vs slower dual cards for a gaming machine, unless you are doing 3D rendering then it is actually slower dual cards over single faster card. gaming = single faster over multiple slower 3D rendering/Animation = multiple slower over single faster There are many reasons for this, for example when rendering a full 3D scene in a 3D program the video card will be locked up, meaning that your system, no matter how powerful will increase input lag, but if you have multiple cards you can dedicate some cards to rendering and others to the screen to continue your workflow (render a high end scene + edit a model at the same time) . When I had my 3D rendering machine it worked wonders, I could allow a full scene to render for hours and still use my PC as normal, even play games on the second GPU! oh the memories! I am waiting for the day I build my dream PC again (5 more months to go)
