I have a GTX750Ti and have been offered a GTX650Ti for free

[cretinbob]

Now that I have your attention, I understand that asymmetric SLI is possible IF both cards are GT or GTX, but you can't mix GTX and GT.

Am I understanding this correctly?

I do some gaming and am good with the performance I have now, but I do even more Photoshop and After Effects, which the extra CUDA cores will help.

The Socratic method is a good method.

 

[xorbe]

Maybe try this:
http://www.techpowerup.com/forums/threads/sli-with-different-cards.158907/

 

[Flapdrol1337]

Mixing kepler and maxwell cards won't work for sli, and 750Ti's don't do sli anyway.

Guess you could put it in for physx or other compute that is able to use multiple gpu's.

 

[NTMBK]

You can't mix the two. Tell your friend thanks, but he should give it to someone else/sell it.

 

[Seba]

Get the free card and then sell both and buy something better.