Verified answer

Yes, you can use two different graphic cards with SLI not plugged in but you can't have 3 cards in SLI with a forth not being in SLI. In general , I would say that 3 titans are more than enough regardless.

Yep, use the less powerful one for a PhysX Card. You will not get as much of an increase in performance, but it will be noticeable especially in games that support PhysX, like Borderlands and The Witcher to name two.

Yes you can. You can use the less powerful one as a dedicated Physx card

it wouldnt work in sli anyway

yes