8 core mac pro

There is a literal library of info on the front page for these questions. But summarizing:
The 6-core is mathematically and always faster (by a ton or by a small margin depending on task and software) unless you need to utilize more than 32GB of memory on a regular basis. A 560ti is not much of an upgrade if you ask me. Get a 570 and now you are in business. But they are not officially supported and have caveats. See every post on the subject on the front page.