I ordered the base 27" and just upgraded to the 3TB fusion drive.
I was tempted to max out the GPU and CPU as well as it's only an extra £150, then only an extra £200, and then only an extra £130.....
In the end I decided that the extra money will be better spent on a new iPad, iPhone, or perhaps a new Sony RX100 camera.
One other reason I resisted the upgrades was a review I read, the reviewer had maxed out the previous iMac model (Late 2012) but found that the extra heat meant the fans ran much more often than his previous iMac. I'll be moving from a MacBook Air so I've come to appreciate silent computing so I decided to avoid adding lots of potentially hot upgrades to the machine.
Hmm. If i was told that before i may have aimed for the i5. I actually started a thread about it but ppl said there was hardly any difference in heat.
I was going for the top graphic, gtx 780m, but was unsure about the cpu.
The machine was expensive at it was so i throw in a i7 cpu aswell.
Silent computer is a big deal for me.