These 4 graphs really say it all:
For most of us 2 dual GPUs are better. It seems elsewhere in the world (Brazil, Russia, India and China), the trends are different though.
A Global Phenomenon
"Ted Pollak, Senior Gaming Analyst said "We are witnessing the market morph and grow into different areas. This has always been a strong point of PC gaming; the ability to adapt to different entertainment environments and requirements. The hobbyist aspect of the DIY market is driving billions in component sales and small form factor rigs are being hooked up to HDTVs, essentially being used as 'super consoles.'"
In its 33 country analysis of the gamer market, JPR has found very strong demand in the BRIC countries for systems, accessories, and upgrades approaching $4.7 billion in 2012 and growing to $7.7 billion by 2015." ~ TPU
For the first time since HD4870X2, someone can fit fastest single GPU in SLI (on 1 card) into a small case, and in fact even 2 of them using a nice small factor board.
Lian-Li V354 Case
This case can easily swallow an HD5970 or GTX690 (or 2).
^ I can see how the market for smaller cases may become more appealing for enthusiasts in the next 4-5 years since a smaller case doesn't look ridiculous in the living room next to a nice plasma/LCD and in many places in the world condo space is at a premium!