I already know about the driver hacks that need to be employed if you want to use an ATi card for graphics and a nVidia card for PhysX; I have a different question.
I have an Asus GTX560Ti with which I am entirely satisfied for games and PhysX. There is some GPGPU processing I'd like to do in the same box using one or two ATi cards. These cards will not even be connected to a display. Will this work, or will their mere presence kill PhysX (or worse) on my GTX560Ti?
Mobo is a 990FX Asus Sabertooth with 16GB of RAM and a 970BE CPU. GPGPU process to be run is bitcoin mining, which is not feasible at all with nVidia cards. I will add one or two HD6790 or HD6770 cards for this purpose.
Worst case, I'll forgo PhysX and sell the GTX560Ti, and step back to the HD6790 I already have for my games (which aren't the latest demanding FPS titles), then add the other card(s) for GPGPU.
I'm not sure that proper drivers would need to be installed for GPGPU computing because you're right, they're not displaying anything or rendering anything. It's just computing power. However, the conflicting video cards might cause issues, however there's no way to tell.
If you already have a spare 6790, throw it in there and try it out.