There have been reports of people enabling Physx's on the GT200/G92/G96 and even the G80 NVIDIA cards.
The new Physx Software can be found here.
Only NVIDIA drivers 177.39 and upwards work with the Physx driver.
177.79 WHQL Candidate Drivers:
177.79 - Windows Vista 32
177.79 - Windows Vista 64
177.79 - Windows XP 32
177.79 - Windows XP 32
Most Physx gamers seem to work although, Cellfactor is one that many people cannot get to work.