I just bought a 7970 for that exact reason - Mining during the day, and gaming after work. The problem when deciding is there are too many options: Do you want a regular or GHz edition? Reference or custom cooler? What brand? How much do I want to pay?
I ended up chosing
this one. It's pretty much the most expensive one, but it's worth it, imo. Gigabyte has always been a rock solid brand. It's factory OD'd to 1100MHz. I prefer reference blower coolers, but the tri-fan setup seems to perform very well.
Buying a non-GHz option is cheaper, but it's very hit or miss how far they can OC. A GHz edition already has higher clock speeds, and is much more likely to be able to hit 1100MHz or higher.
They are cheaper options, but I tend to stay away from Diamond, VisionTek, XFX, and HIS. I just don't really have as much faith in those brands.
I like Gigabyte, PowerColor, or Sapphire. I've heard Asus and MSI are also good.