Reading this Radeon HD 6950 CrossfireX review shows charts with multi-GPU's performance results. A pair of 6870s is a viable option for you and will give excellent frame rates in games past, present and future.
Tri-fire might be overkill, but kick a-s. As for out dated goes, the pace of tech now a days ----- but you know the rest.
In my opinion you should be good with crossfire for a couple years.
Oh an the link; http://www.guru3d.com/article/radeon-hd-6950-crossfirex-review/