I have a 4890 but my psu isnt made for crossfire. To buy a second 4890 ($220) + PSU ($125), I am looking at about $350 without eyefinity or Dx11. Also, the scores we are seeing for the 5870 are with immature drivers.
If I didnt have to upgrade my power supply, I would get the second 4890 in a second, but alas, i have no choice. Also can't get SofTH to work no matter what for some reason.