^+1
I just got a second HD4870 1GB for myself. You can get them now fro $169., and the 512MB version for $139.00 so if you dont mind paying $50 - $80 more for 6 - 10% Performance increase get the 4890. Your eyes wont notice the difference.
Crossfire setup will be $100 -$160 Cheaper, depending if you go with 512MB or 1GB version of the hd4870.
Cheers