My initial upgrade plan was another 5870. That was something I wasn't too eager on because I also would need a new PSU. Anyway, I soon found out that on such large resolutions its mainly vram holding back performance. Any AA on Arma 2 is proof!
FOr now its off.
So I would like a new card with lots of vram. Enter GTX 580 with 3 Gb. Excellent. The Google tells me Nvidia only supports "Surround" screens with SLI.
Back to scratch.
So for now its looking like the monster 6990 is my main contender. I'm also considering a 2 GB 6970 so I don't have to deal with Xfire troubles.
Thanks for the input guys!
EDIT: Yes, I think you are correct, Arc. And very nice cards, Jan!