It will be by default with that rig, you have to manually disable it. You'd get more like 70/80 in dx9, too.
70-80 seems awfully low for a 5870 in DX9 mode.
I get 45-50 with a 4850 with max settings and 4AA.
I haven't needed to turn crossfire on for this as the framerate with just one card is more than good enough.

