After much research a few months ago, I found the best card for the money, with regard to Final Cut editing was the AMD 270x. The Bruce X benchmark performance was about 30s, which ranked it quite high compared to several others.
If you can afford it go with the 280X rather than the 270X. The 280X is faster still. You can also put dual 280X (or dual 270X) cards in later and get even faster render speeds. You cannot mix 270X and 280X cards to get more performance. The 280X performance is approx. the same as a 7970 (the 280X is a little faster) and is about 2x the performance of a 5770.
Choose your 270X or 280X card carefully as there are differences, mainly on the video output options. Some have dual DP or dual DVI. I have twin 280X cards each with dual DVI and it worked out of the box. No special flags were needed for installation and no drivers needed. You do need to do the EFI mod to get the cards to boot. This is 10-15 mins (max) of work and is trivially easy.
I'd also forget about OWC RAM thats 'special' for Macs. You have PC hardware, get the right RAM for the motherboard. Check, check and check again that its exactly the right RAM. Most memory issues are due to people assuming that just because there RAM says (for example), DDR3, 1666Mz CL11-1-2-1 it will work in their motherboard. RAM and motherboards are sensitive, spend the money and get the right RAM and save yourself an enormous headache. Don't skimp here.
Also unless you absolutely need to, dump the wifi as moving video around by wifi is like having teeth pulled with no anaesthetic and without the fun. Video needs 1Gb as a minimum and 10GB is even better (though f£$%^Y$RTYing expensive.
I don't water-cool and use a Noctura cooler. Its cheaper, just as quiet and less chance to go wrong. You have different CPU's though so you may need that.
RAID-0 is brilliant right up to the moment it goes wrong and you lose both your disks. As somebody else has said, don't do that or if you, make sure you spend some money on a decent backup system. Ask yourself the question how long can I afford to be without my system when I need to rebuild, or how much will it cost me to redo all my FCP X edits when a disk throws a wobbly. I use my Mac for development purposes, though not for commercial FCP use, and I have Chronosync doing daily backups, Time Machine for immediate backups (though I don't trust it at all, hence the Chronsync) and replication off to another machine on an daily basis. You have to judge what would happen when things go wrong because they will.
Best wishes,
Rob
I have to admit that I didn't read through the entire thread as it's 155 pages long 
I'm just wondering since the build was made in April 2014 a quick bullet list of what users would recommend doing differently as there are surely updates or advances since then...
