Assuming the Acers are 1080p, I feel that a 6950 2GB card may do better that Cross-fired 6850's. The extra GB of video ram will help a 5760 x 1080 Eyefinity display run smoother on higher game settings. This would leave you with an added ~$120.00 expense and an extra 6850. But would also allow better GPU cooling and eliminate any Cross-fire issues that many high end gamers have experienced.
Also, since the 6850s' Crossfired would only yeild 1GB video memory, you could try the single 6850 first, and see how that goes.