Supporting the latest version of PureVideo HD (a first for a very high end NVIDIA board) and Quad SLI (it'll be possible to pair two 9800 GX2 on an NVIDIA platform when the drivers become available meaning next week), the 9800 GX2 is the first card to support HybridPower. HybridPower is the most interesting of the two Hybrid SLI technologies (with GeForce Boost for entry-level boards). It can totally deactivate the 3D card when using office applications, thus only using integrated graphical circuits in the event where a 790i is on the motherboard. Even if 3D cards have made some progress over the years, they still waste much energy when you're not running 3D applications, which is most of the time for many users.
The GeForce 9800 GX2 has some additional components that wake up a deactivated card (NVIDIA refused to say more). Requests to activate or deactivate a card are sent through the SMBus on the PCI Express bus. This is the first time this bus has been used and a graphics card must be compatible to respond to the requests. It must also have upgraded power supply stages and the capacitors, so it can react very quickly. Even though we weren't able to check since we did not have a compatible platform, we were able to see that it did work well a few months ago with beta drivers. At that time, activating or deactivating the cards could only be done manually and there was a delay of a dozen of seconds.