On Alice and Batman AC Physx high will run CPU but you need some serious grunt to make it happen, a Q6600 ain't going to cut it. When running on a single 580GTX with a overclocked 3960X running Physx on CPU is faster than forcing the 580 to render and do physx at the same time. People with high end rigs and AMD GPU's don't need to worry so much.

Just so you have an idea of how much grunt is needed, a 2600k at stock won't be able to maintain 60 fps on Alice when calculating Physx, it'll run reasonably well for most parts but the fluid calcs will cause a drop. It shouldn't be too choppy for the most part, but fluid effect is used often enough for it to be a pain.