I'm not exactly sure, but isn't Qimonda the best GDDR5?

Seems the blue Sapphire 4850 is utter crap, I could do 800MHz flat, but 830 didn't work no matter what I fed it with (which was up to 1.58V under a accelero twin turbo). Yes, I'm aware that those kinds of voltages are prone to kill the hardware, but I just wasn't bothered with keeping it alive.

My reference (old) Sapphire 4850 is better though, running 3DM06 at 800MHz with 1.3V right now (just thought I'd start there, haven't tried any less voltage).