I use FRAPS for monitoring the fps. Also, I might add that I play in 1440x900 resolution which is a little lower than most people play in. When I do have drops in frames it's when I spin around wildly with tons of npcs on screen. As well, my preferred playstyle it to get distance and pick enemies off and not be in the center of mobs, I'm sure this also makes a difference.
I did not know the option was available for PhysX in the menu now. I set my options on game release and I haven't changed them since then. On initial release I did not see an option to change PhysX as you say there is now that option available, but either way... it's also in the ini file.
You know you can use Afterburner (free & swiss armyknife for any GPU) with more options for optionally monitoring (OSD/LCD keyboards) your GPU usage, temps, clocks, fan speed, volts, VRAM. I also use Fraps but toggle them on one at a time when I'm not recording.
Has anyone noticed improvements with the Nvidia 310.33 drivers in Caustic Caverns? If your playing happily enough, post your fps+GPU/s+drivers.