Quote - I have the 27" iMac with the i5 Quad.  It runs Carrara like a dream.  It basically blows my old (now dead) C2D PC out of the water.  As far as screen size, it is a personal preference, but I love the size.

It comes standard with 4GB of RAM, and can be upgraded to 16GB.  I would recommend doing that yourself as it will be cheaper than buying it from Apple.  The RAM slots are underneath the display where the speaker grill is located so it's an easy upgrade.

I don't think you will be disappointed if you go with the 27" iMac with the quad core.

I think so :)

 
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Holyforest,
Hundreds of shaders for Carrara