AutoCAD for Mac

[#1]

I'm about to purchase the new 27 inch iMac to use AutoCad 2013. I will be using AutoCad 2013 to design audio/visual systems for various buildings. Here are the choices for graphics cards that I can have installed: 

NVIDIA GeForce GTX 660M graphics processor with 512MB of GDDR5 memory

NVIDIA GeForce GTX 675MX graphics processor with 1GB of GDDR5 memory

NVIDIA GeForce GTX 680MX with 2GB of GDDR5 memory

Which one will do the job for the type of work I will be designing on auto cad??? Obviously the 680 is the best one but it's also the most expensive. Will the 660 be sufficient for the kind of work I want to do?

[#2]

Are you planning on a lot of 3D renderings with AutoCAD? Are you adding more than the 8gig RAM default? Pony up for the faster Flash Drives?

Or do you do just 2D drafting and line drawings all day long?

There are many ways to make AutoCAD faster, graphics card is not the main source.