And you think you need dual GPU to run PhysX?

Depending on what resolution and what detail settings you chose, you can run PhysX on a single card with good or decent frames.
Driverheaven showed that you can run Batman AA with maxed out physics on a single GTS 250 at 1680x1050. While they had playable average frames, their low minimums would have made the game lag a bit though (especially in the Scarecrow levels), but keep in mind they were running the game using maxed out graphical settings, with 2xAA as well.
If they disabled the AA, the performance would have increased significantly.
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