What is a scratch disk? Is it something I need to purchase and setup?Pick whatever screen size you want. Then add on 16GB if getting a 21 or 32 if getting a 27 inch. Personally I would do a SSD and a Thunderbolt scratch disk. Otherwise, you could get a hybrid drive to ensure great speed.
You mean the 3.4GHz i5-4670, don't you?Personally, if you can swing the 27" iMac then go for that - its a gorgeous screen.
I'd go for the 27-inch: 3.2GHz with 16Gb of ram
Here's how the i5-4670 stacks up against the i7-4771: http://www.cpu-world.com/Compare/437/Intel_Core_i5_i5-4670_vs_Intel_Core_i7_i7-4771.htmlThe i7 would definitely be a nice boost in power. However, the i5 is no slouch by any measure. I have used both iMacs and I personally like the 27", but everyone has their own preference. In my opinion the 24" they used to make was the sweet spot.