You could add this card in for physx without issues. It's got way more than enough transistors for physx. In other words, you'll probably never see it over 50% usage even with the heaviest of physx games.
Also, most physx games aren't optimized to use an entire Gpu especially if it's DX9 like Hawken even on a 64bit OS.
You could get away running pretty much any physx game out there with as little as a GTX 750, Your GTX 670 would just be over kill on power consumption.
Back when PPU was trying to be lifted off, it was simply that. It was a 500mhz dedicated chip that was capable of physx. It was a little lacking in horse power even at it's debut time, but really close to all you need in physx processing power. The Asus Physx P1 card was bundled with Ghost Recon Advanced War Fighter.
So really your 780 having plenty of Cuda cores can handle physx on it's own. Now depending on your games resolution and quality settings is where impact comes evolved. If your maxing the 780 with current settings while physx is turned off, it may be a good idea to offset the load of Physx to a dedicated PPU/GPU/CUDA cores. Your physx also uses Vram so this can be taken into account while debating what the gpu load is during gaming.
To conclude whre I'm headed with this stuff is simply, yes your PSU will handle the 670 for physx. Simple because you'll have way over kill on cuda core count and plenty of Vram.
Bare minimum however would be 512MB Ram and 384 cuda cores, essentially a GT 740 would do the trick, I ran a GTX 750 myself. Worked out great. Low power and great eye candies.
Long Live Ageia - Cell Factor Revolution. Now this game can tax a PPU to death. Current GPUs with GDDR5 and plenty of Cuda cores can get this game running rather smoothly.
