I am getting ready to order tonight! 21.5'' iMac with the Fusion Drive and 16GB RAM. Since I am spending so much already I thought about upgrading to the 3.1GHz. Is the speed/difference really that noticeable? This will be my first iMac and I will be coming from a 2.26GHz Intel Core 2 Duo Macbook Pro so either one will be an improvement.