iMac help, please - Graphics

[olds]

I have a friend (lives 3000 miles from me) that knows nothing about computers and has an iMac. I don't know which one, how do you tell the model?

Are the graphics cards replaceable in an iMac? I have a NVIDIA 660ti I'd like to give him.

TIA

 

[Tyranicus]

No, they are not. The GPU is part of the main logic board.

 

[olds]

Thank you.