I ordered a new PC which is gonna have a GTX580 once everything arrives. My current PC has a 8800GT. Should I use the 8800GT as a dedicated PhysX card? Or will that somehow deliver lower FPS's (I read somewhere that if you do that with a low-end card that it will have trouble because the 8800GT can only work so fast, causing the higher-end card to slow down to allow the 8800GT to keepup)
Is this true? If not, should I add the 8800GT to the PC?
I don't think It'll be in "SLI" just as a dedicated card under the PhysX Menu of the NCP
Well, you can try that later when your PC arrives.
Seriously, a single GTX580 is more than enough to maxed out almost anything even without PhysX card. Try that and see it for yourself whether your fps is increase or decrease...