Best nvidia card for i mac 27in late 2009?

osx 10.9.3
processor 3.06 core 2 duo
16gb ram
ATI RADEON HD 4670 256mb

I need a new card! Is it possible on this machine to get a 4gig nvidia graphic card? Do i have any options? Ive been thinking about building a pc but would first like to see if i can salvage my mac since all my programs are mac.
Also if there is a card and still does not work out i can just transfer that to the new pc build. I love blender and really want to use cycles and speed up my renders. I plan on using it for modeling and animation.

If i did build a pc i noticed it said have an 8 core processor and the only option i could find was the intel core i7 5960. would this be good. It is expensive but i would buy if i had too.

AMD i know nothing about so i assume they have more 8 core processors any help is greatly appreciated thnx!

I need a new card! Is it possible on this machine to get a 4gig nvidia graphic card?
No you can’t

Ok thnx! Looks like i better start putting a new machine together then.