Fusion? Graphics Card? Processor? Need help understanding what is best for me.

Here is an article that does a good job of answering your question.

http://www.macprovideo.com/hub/final-cut/final-cut-pro-x-basic-hardware-guide

I have the newest 27" iMac with the i7 processor, GTX 780M GPU, and 3TB Fusion Drive. I got the stock 8GB of RAM and added 16GB of Crucial memory for a total of 24GB.

Final Cut works great on this set-up. I'm just not sure which of the upgrades is providing the best performance upgrade so I would go with what that article mentions... choose a GPU upgrade first over a CPU upgrade.

Note: I would never get a computer again without a Fusion / SSD drive. I have recently replaced the hard drives with SSD drives in an older 2009 iMac and 2009 MacBook Pro (my son uses these). He said it was like getting a new computer. The start-up and application launching times are about 1/3rd of what they were prior to the SSD upgrades.

For video work, I'd suggest the fastest processor and most RAM you can afford. If you're buying a 27" iMac, you can add RAM later (buy it from either OWC (macsales.com) or Crucial and install yourself - much less expensive). The graphics card is more important to gamers than video work. I have a 2010 iMac 2.93 GHz i7 and it works great using FCE, Aperture, iMovie, iDVD, etc.

Since you need to have at least as much free space on your hard drive for proper rendering as you have temporary files in your project (more is much better), I don't know how well a Fusion drive would work. My last 1 hour project collected about 150 GB of HD files and I always keep at least 300 GB of free space because of that. I don't know how/where the fusion drive would keep such files and/or which drive it would use to render. I'd love to have an SSD in my iMac (I have one in my MBP), but there is no way I'd be able to justify a 1 TB SSD.

I also have Motion and while I'd appreciate the power of a Mac Pro with 64 GB RAM, a 1 TB SSD, and 2 graphics cards, my iMac handles it quite well. It's not lightning fast, but it works.

As for the Fusion Drive: as I said, I don't know the answer on how the two physical drives work together in assigning space to certain files and a 256 GB SSD may not do unless all the project files are being kept on the HD.

A Fusion drive or an SSD makes the computer operate so much faster for routine tasks such as booting up and launching apps. The boot time for my son's 2009 iMac was 50 seconds using the hard drive that came with it. When we installed an SSD drive instead, it booted in 20 seconds. We had similar results with the 2009 MacBook Pro before / after the SSD drive upgrade.

With a Fusion drive on an iMac you most likely will have the operating system and your applications on the SSD partition. Your working video files for FCPX would usually be on an external drive (or you could keep them on the fusion drive).

My only concern is the cost to replace a fusion drive. I don't know how much Apple charges for this compared to a normal hard drive. You may want to inquire.

Whatever you decide, be sure to include Apple Care in your purchase price. These machines are not very easy to open / fix yourself and something like a failed logic board costs about $800 to replace (had my 2012 iMac logic board fail after 14 months). As another tip, pay for it with a credit card that provides an extended warranty. My American Express doubles / extends the warranty up to a year. So I end up with 3 years under Apple Care and another year under the Am Ex extended warranty.