I have got my gaming custom pc. I've got 4gb geforce gtx 770 as a graphic card (pc is made for gaming and some overclocking). Should I put my old graphic card from my previous pc to have even better performance (my previous pc wasn't made for gaming and overclocking at all). And does old card will allow me still to overclock.
No. Assuming your old card is also an Nvidia card, they can only be put in SLI if they are the same model. You could use it as a non SLIed dedicated Physx card, but from my understanding its not really worth it.