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.
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Shouldn't slow it down at all.
If you set the card to only run Physx, it will do just that. If the game dosn't support it, the card will just sit there idle. You dont have the worry about bandwidth. The only downside is power draw.