So I just ordered two Gtx 460's off Newegg and I was wondering...... If I use one of the cards for PhysX will the performance still be the same? In other words, say i had 100fps with the two cards going without one dedicated for PhsyX. When I turn on PhsyX for one of the cards will the framerate go down or stay the same?
If you turn one into a dedicated PhysX card, then you will get the frame rate of having a single card, you will not get SLI performance at all.
However, Nvidia allows you to have SLI and choose which card to have PhysX performed on. In this situation, you will have full SLI performance when you don't use PhysX, and when you do, one of the cards will do both PhysX and use the left over power to help render in SLI.
Normally this shouldn't be a problem and isn't for most users. Go ahead and enable sli once you get them and enable physx for the GTX460. The load will either balanced across both cards or on just one but will be still run in sli so it won't be a problem. I have gone through a few sli rigs in my time.