I got about a 90% increase in performance with my second GTX260. In fact it scaled very well in almost all games. Crysis doesn't use PhysX so no, a PhysX card would be useless for Crysis.
EDIT: GTX285 single vs SLI in Crysis...
Single: avg: 20FPS
SLI: avg. 38FPS
Yup, scales very well!