Bfg 9800gtx+ OC vs SAPPHIRE FleX 6870

Hello,
building a new 2600k rig and really want to know if im going to see a real big upgrade i choose the sapphire card because i want to do an eyefinity setup and the flex edition i thought was nice because you dont need active adaptors if you dont have display port monitors and i belive eyefinity would be nice but would rather have raw overall prefomance over a 3 monitor setup i currently have a 25" and a 23" set up so eyefinity would only need an extra monitor but really want to know if its really going to improve my fps at full settings grabing another 9800 or going to a dual 6870