I've just started, so, no overcloking yet.
By default it gives ~20MHs for ethereum.
Windows drivers don't support >8 cards. So I'm trying to use nvOC which is based on Ubuntu (remote configuration not working for me currently).
you can run 8 cards on windows if your mobo supports it of course.
About the mining cards I hear people are avoiding them for their low resell value, also I don't see the point paying the same for 470 4g as for 580 8g