All these posts yet no one mentions the good ole Mac Pro LOL. How about a 2.8ghz quad 5,1 Mac Pro from 2010?
BTW if you are looking for graphics card choices for your Mac Pro, you can get non-Mac cards that have been flashed to work with Macs at MacVidCards.com. That's where I got my GTX 680 and it's great with Adobe Premiere, etc.