Well,
I have my Crossfire 4870's in place and it does get me another 2500 or so 3Dmarks. Unfortunately, gameplay (Bioshock) is slower and choppier than with the single 280GTX. Although the install with the 8.6 drivers was a lot smoother than my attempt with the 3870's I tried earlier. The problem I always have is the question of whether or not Crossfire is functioning or not in gameplay, and correctly. I'll do some more reading and decide which one to stick with, but I really prefer the single card solution.