Hi, I am using a GTX 560 and a 9800GT. The 9800GT is a dedicated Physx card, now when i am gaming on Physx compatible games i notice better FPS so happy days. So the question is when i am playing games that dont support Physx will having the 9800GT sat there doing nothing slow down my main GPU as it may take up PCIe bandwidth. I think if you sli 2 compatible cards they run them both at x8 but not sure if the same applies for a physx setup. I have 3 PCIe slots and i use the top one for the main card and the second slot for the Physx card. I am fairly good with PC building but have little to no knowledge when it comes to Physx.
Dont know if needed but am using an Asus P8Z68-V Pro, i5 2500k OC to 4.7GHz, 8Gig ram 1600MHz, Gigabyte GTX 560 Which i OC the core from 830 to 910 and a 9800GT for physx.