Thanks. So you feel the stock i5 on the 27" is good enough. What is the advantages of i7 then?
This might be too little info too late, but the Core i7 can hyperthread while the core i5 can not. That means the Core i7 functions as an 8 core processor, while the Core i5 functions as a 4 core processor. I'd spend the extra couple of hundred bucks on the Core i7. The Core i5 is quite old at this point, and I'm surprised Apple offers it in a newly revamped machine like the iMac.
The i5 is a great inexpensive alternative. But honestly for photo and video, just like a previous poster said, you'll the good everything. GPU should def be atleast 1GB. You can definitely get by with the i5 if you don't have the money after getting the GPU, but if you can, get both.
Why does everybody always say a super duper graphics card is mandatory? In both Lightroom and DPP it doesn't help with rendering. I guess if the software you use can take advantage of it, that's one thing but CPUs are king of the hill when it comes to RAW conversion.