If two slots just weren't enough, take a gander at ASUS' ROG Matrix GTX 580.
Apparently factory-overclocked at 816MHz, 1,632MHz and 4,008MHz for the GPU, shaders and 1,536MB GDDR5 memory, respectively, this behemoth boasts a 19-phase VRM for power delivery. There's also word of a (vanilla) version coming in with core clocks of 782MHz.
The cooling solution used is said to be based on ASUS' DirectCU II technology, and there are a host of ROG features too - mixed with the usual mix of bells and whistles such as voltage read-out points and onboard button wizardry.
If that isn't enough graphics firepower for you, there's always the MARS II GTX 580 - a dual-GPU SKU that looks set to feature the same power delivery credentials as its sibling, and some serious oomph to try and overcome the HD 6990.