ATi's had a rough time of it over the past year or so with every single one of its last generation cards failing to live up to expectations and nVidia's equivalents consistently beating them on performance. However, with the release of the HD 3870 and HD 3850 at the tail end of last year, ATI looked to be on the up again. With decent performance, unbeatable features, and competitive prices these cards made for a worthwhile alternative to nVidia's mainstream and enthusiast products.
Unfortunately, while the big money is to be made in these mid-range (£50 - £150) markets, something called the halo effect means that having the fastest card available still has significant influence over the general public's buying habits. Put simply, if someone hears one company makes the fastest card available, they're going to assume the other products the company produces are also the fastest in their respective sectors. The fact this mentality is completely ridiculous causes much frustration for manufacturers and journalists alike but there's nothing we or they can do about it so manufacturers will always seek to gain the performance crown just to convince these people.
The problem for ATI is that modifying or augmenting R670 (the chip at the core of the HD 3870) to make it significantly faster would require a complete redesign of the underlying architecture and for a number of reasons this is something it simply isn't in the position to do.
An architectural redesign requires a significant investment in research and design and rushing this would cost large amounts of money and potentially lead to mistakes, which would be even more catastrophic than not releasing anything. Also, releasing a completely new chip now would've cut into ATI's plans for its next generation of hardware and potentially disrupted sales of cards based on what would be perceived to be its 'old' architecture.
So, if a brand new chip wasn't the solution, how could ATI leverage its current capabilities to create a single competition-beating graphics card? Simple, it stuck two R670 chips on one card and ran them in Crossfire. The result is called the HD 3870 X2.