Though it depends a lot on what resolution you will be using and what you're going to be running, at that speed I would think an HD 7870 would be a good bet.
I do think at higher resolutions with a 7900 series card you'd run into a few walls with games that are more CPU dependent, but I welcome discussion on that.
PCI Express 3.0 is backwards compatible with 2.0, so there is no reason to seek out older hardware based *solely* on that.
If you want to go the nVidia route instead, wait for someone else to weigh in as I don't believe I have enough knowledge about their products to offer you a good recommendation.