AMD's new Radeon HD 6990 video card reclaims the top spot in their overall GPU product hierarchy. There is no single-GPU card that can compete with two Cayman-class Radeon HD 6870 processors running in tandem. This was also true for the prior generation's king, the HD 5970, and at first glance the 6990 looks like a one-for-one replacement, but is it? Not really; it's better, or I should say it's more better. The PowerColor Radeon HD 6990 takes a slightly different tack, as the full complement of shaders (1536) is always available from each GPU. There is a BIOS switch that unlocks the core clock and the GPU voltage for the more adventurous among us. Normally that kind of enhancement-on-a-switch might seem a bit ho-hum, but with the power of 5.3 billion transistors on board, there are physical limits that must be respected. Heat and power go hand in hand, and some say it's folly to try and pack this much power into a single package. It's better to have two cooling systems, and two slots, and two sets of PCI-E power connectors to spread the load, they claim. They may be right; we'll just have to test it and see... Follow along with Benchmark Reviews as we put the new PowerColor AX6990 4GBD5-M4D to the test.