Gigabyte HD 4850 GV-R485-512H-B Review
 

This card was a bit tough to overclock because the temperatures held it back. I was able to get the card from the stock GPU clock speed of 625MHz all the way up to 675MHz. The memory went up from the stock frequency of 990MHz up to 1075MHz. Overall, that's a pretty good overclock, especially since this card was overclocked 35MHz more on the GPU and 30MHz more on the memory than the PowerColor HD 4850 that it's competing against. With an aftermarket cooler, I'm sure this card would be able to go much higher than what I got it at.

http://www.overclockersclub.com/reviews/gigabyte_4850/