I had a MSI Twin Frozr III for the longest time, and it overclocked like a champ. I can't remember the exact clocks, but I know it was around 1200MHz, and some go higher than that if you're lucky. If you can get one with a 7970 board like mine, those chips can really go above and beyond the others while staying relatively cool and reasonably quiet. I think Sapphire also had a 7950 with a 7970 PCB as well. Otherwise, the other cards will probably OC a tad lower, but for the most part do pretty well.