Hybrid SLI and physx

I just enabled Hybrid SLi with a 9800gtx+ and a 750a motherboard. Meaning I see two GPU's in the Nvidia control panel, and meaning I have two options to run PHysx on, normally i just run it on the GTX+ (I know it only works in a few games, but what the hell). So would it work to have the 750a run phsyx, and if it does would it be better to run it on that, the gtx+, or would I get better frame rates without it. (namely on crysis, AC1, COF, and AC2 soon)

I'm gaming at 1650x1050 and with a phenom II 945

Also how can I be sure hybrid sli is running, I enabled it in the BIOS and the manual says a sli driver will install, and an icon be viewable on the bottom right of the scree. The driver installed and than was it, no options, nothing.