I have the 2011 Mac Pro 6-Core with 14GB of RAM and just installed two Samsung 1TB Raid-0 SSD's and it rolls like a champ. It's definitely worth the investment in computing power. If for no other reason than your sanity. :)
You'll definitely need the Raid-0 SSD's as a starting point. You can get 1TB's for around $500. My write speed is more than sufficient for 2.5-4k editing. I run FCPX, Resolve, AE, and many other programs without a hitch.
Just as a note - if you go with the 2011 Mac Pro and Apple Display, make sure to get the Cinema Display, and not the Thunderbolt display! It doesn't work. If you get the new Mac Pro, it's not an issue.