Fusion? Graphics Card? Processor? Need help prioritizing for video editing!

CPU and GPU are what matter. Next, is ample RAM. Get at least 8 GBs but more can't hurt.

Since the speed of the CPU is dependent upon which model you choose then the GPU is pre-determined. If you get a 21.5 inch model, the fastest CPU option comes with only one GPU option. The two 27.5 inch models come with the same GPU. So the GPU you end up with depends upon the particular model option you choose. So essentially if you want the best GPU buy the top of the line 21.5 inch or either 27.5 inch model.

Suffice to say that the discrete GPU is better than the integrated IRIS GPU.

Options to choose — in this order —

Get the NVIDIA 780M - it's 2 GB MORE vRAM and almost 30% faster than the 775 ; a lot of processing in FCPX and Motion are being redirected to the GPUs.

Get a Fusion HD — it's almost as fast as the Flash Storage (aka: SSD) and not nearly as expensive

Buy your extra RAM from Crucial(.com) -- you'll save $40 and you can ADD it to the basic 8GB RAM for a total of 24GB RAM (installation is super easy!! * I'll get back to this.)

By all means, get the i7. It appears to the OS as an 8-core processor (due to "hyper-threading" which you don't get with an i5.) Even though they're clocked at almost the same speeds, the i7 performance is much better.

*Installing your own RAM:

There's a button inside the "port" where the power cord plugs in -- you press that down (as far as you can) and a door on the back of the 27" iMacs opens up. There are two clip-like things that you pull up and out slightly. Insert your RAM cards into the 2 vacant spots (they are ordered 1,3 and 2, 4 -- with the original 8GB in the 2,4 slots.) Push the cards all the way in until you hear a click. Return the clips to the original position and put the door back on. You're done. Plug the mac back in and restart. Check Apple menu - About this Mac to make sure it reads 24GB RAM:

Easiest Mac DIY ram install ever!

If you can afford the the GTX 780 GPU, then that is the option I would select. Take the model with the least RAM pre-installed because you can upgrade the RAM yourself for less money, and it's an easy DIY upgrade. The $200.00 saving will offset the $150.00 for the better GPU. For your kind of work you don't really need a Fusion drive. If you were to upgrade the storage I would go for a larger hard drive - $150.00 more for the 3 TB HDD.

Well, that's what I did (forego the Fusion drive.) However, I don't think for a minute that you would regret breaking your budget slightly to include it if you can possibly stretch it. "Brown bag it" for awhile to make up the difference. My general advice in the past to friends of mine making the leap from PCs to Macs (even though I don't always follow it myself) is: buy what you want and don't look back. The Fusion drive will put your new iMac (along with the other options) in the ballpark with the New Mac Pro in terms of performance. Just something to consider. I can live without the Fusion Drive, but I think I would have really liked to have had it.