Club3D Radeon HD 7970 royalQueen with 925 MHz GPU, running at 1160 MHz on top of a PCI express 1.0 x16 lane. Average hashrate 650 MH/s.
Edit: I suggest buying a 7970 with the GPU clocked higher in the factory than 925 MHz. Forgot to check my MHz when I bought it

but I still expect to earn back the money in at least 6 months, being somewhat sceptical.