So if you plan on multiple-screen gaming, are you better off with the twin-GPU HD6990 or the nVidia equivalent Zotac GTX590?
In some degrees it depends upon which game you want to play as some perform better than others in certain circumstances. However looking at the results in an overall manner we'd definitely suggest plumping for the Zotac GTX590.
In the single card scenarios, either with a single screen or the triple-headed monster, the Zotac comes out on top more often than not, and by quite a margin too.
The new 275 drivers from nVidia deserve special mention as the difference between the results we obtained using the 275.33s and those we saw when we first tested the GTX590 on the 267 ForceWare drivers is just incredible. For example our initial Metro 2033 tests saw 66 FPS, today on the 275s we got 117. Now that's the extreme end of the scale but even so we saw improvements everywhere. It seems nVidia have finally unlocked the insane amounts of power available from the GF110 GPUs.