FCP X is designed to work with the AMD GPU's. The ones that appear to work best at the moment at the are the R280x (which is a more up to date version of the 7970). The nVidia cards work very well with Adobe Premiere I am told, I have no idea as I only use FCP X.
The R290x appears to have issues as does the R295x. These may or may not get resolved in Yosemite, but my money is on NOT getting resolved.
I use two 512GB SSD's for my main work and use a 1.5TB disk for slower work and stuff that I don't immediately need. That model works for me, but my main focus on my Mac is software development using very large data sets, a typical data set is 140GB (and by the end of the year it will be 220GB) in one run. This takes 8-10hours to run and disk IO is pretty much a bottleneck, hence the SSDs.
FCP X is a secondary activity but I want it to run as fast as possible. Twin graphics cards (must be the same AMD architecture will work significantly faster than one, so if you can afford dual R280x cards, use those. They need a lot of power so at least an 850W PSU and I would suggest a bigger case. I don't understand why people get emotionally attached to cases and their design, any decent case that is designed by sensible people is fine, but you need to check if it can handle multiple graphics boards AND large PSU AND all the cable routing. I don't water cool, I have a large Noctura fan and its pretty silent. One less thing to worry about.
Start with what you want to do, is it rendering, is it design, is it development and work from there. I am happy with older and dirt cheap 5770's as they allow me to run multiple monitors with zero effort. I run three for development, they work out of the box with zero boot flags and zero configuration, they allow me to run the Bruce X benchmark in 32 secs, which is pretty damm fast and they cost me $50 each. I don't play games, and multiple monitors for development is very important to me as is fast disk (and lots of it). So my machine is great for my usage, but you need to work out specifically what you want to do and build your machine around that. This forum has lots of experts who know what they are talking about (I am not one of them), if you present an informed and sensible question and show that you have researched the topic, you'll get sensible answers from people who know. The worst questions are vague and ask questions that five mins of searching will give you the answer to.
My personal view is that you need to make sure the GPU, CPU, MB and memory all work together for what you want. Hard disk, SSD, PC cases, PSU etc just fall out of the first piece of work. Most branded stuff works very well, if you want a purple backlit perspex case then fine, just don't build your business case around it.
Rob.