Olivier Amrein presents an extensive analysis of GPU benchmark data in order to select the fastest hardware for Cycles.
Olivier Amrein writes:
So you are a cycle fan ? You have an old computer and graphic card, and want to upgrade your hardware to something good enough for running the new cycle rendering engine ? Here is an interesting article for you.
It all started with a cool model and shared .blend file by Mike Pan. People download the file, render it, and add there rendering times in a spreadsheet. It has been posted a while ago and there have quiet some results (around 160 at the time of writing this post).