With many new graphics cards, unless at least one of your monitors has DisplayPort support or is connected using VGA*, an active DisplayPort to DVI adapter is needed in order to get a picture on more than two digitally connected monitors at the same time. As DisplayPort is only found in the most expensive monitors, this is a fact that many buyers miss.
This problem applies especially to AMD Radeon EyeFinity enabled cards such as the Radeon 5xxx and 6xxx series, which enable use of three monitors simulatenously for a combined resolution of up to 7680×1600 pixels, or even six on certain models, for a combined maximum resolution of up to 7680×3200 pixels.
The first active DP to DVI adapters made available were and can still be very expensive, at around $100 each. Also, they have to be plugged into a USB port for external power.
For HD monitors supporting resolutions of up to 1920×1200 (max resolution of DVI Single Link), manufacturers solved this problem by creating an active DP to DVI Single Link adapter, that requires no extra power, and can be had at more reasonable prices.
The problem with these new “passively powered” active adapters, is that they don’t support resolutions above 1920×1200 pixels, while most 30″ and some 25-27″ monitors have a native resolution of 2560×1600 (16:10) or 2560×1440 (16:9) pixels. To support such monitors without DisplayPort connections, you still need a more expensive adapter with an extra USB connector for power. Note that this only applies if the monitor doesn’t come with DP support!
* As VGA is an analog connection, there’s only two digital connections being used.
Here you can see the difference between the new type of Active DP to DVI adapters and a standard passive DP to DVI adapter: