You can get the previous generation Mac Pro towers for a great price still if you look. Its a straight forward upgrade to put in a new CPU (have to get a used one on ebay, but still good) and a standard PC GTX 980 will work if you don't care about the boot screen showing up. Just be sure and get a Mid 2010 or 2012 model so you have the better bus speed logic boards. As for processors you can often get a used HP server with the Xeons in them for less than the bare CPU.