Despite being the fastest graphics card around the CrossFire setup of two Radeon HD 6970s puts it into second place in our benchmarks.
With the Antilles Unlocking Switch enabled you can hit the same speeds as the HD 6970s but there's still no guarantee your PSU will be able to cope with the extra power draw required of it.
It's obvious that the single-GPU power of the Nvidia GTX 580 cannot compete with the current cream of the AMD crop, though it should give a fair indicator of how good the GeForce GTX 590 could be housing two of the GTX 580's GPUs.
In recent tests SLI was hitting close to the 2x performance boost from a second card too...
DirectX 11 Tessellation performance
DirectX 11 Gaming performance
DirectX 10 gaming performance