I've seen a couple of reports in this thread of 4850's getting 80-90 mhash/s. I've been mining on two for the past week and a half, trying to tweak them, but only reliably getting 55-60 out of them. One is on a laptop, so that's to be expected... its gpu is clocked at 500 mhz by default. The desktop's running at slightly over stock, too, but I need to get the card cooler before trying any overclocking. It's currrently running at 103 degrees C. It's an old setup, so the fans could probably use some cleaning. I've read that people recommend underclocking the memory, is that to reduce temps?
Edit: I think it's a cooling problem. I stopped mining, let the GPU cool, then restarted it. I was running at 67 MH/s until it hit about 100 degrees, then it dropped down.
Wow, I think the cards are rated to somewhere like 90C. 100C is really hot. I personally don't like my cards at 80C. I'm new to this but I never use the stock fan control. I either set it manually in windows with the catalyst control panel or use AMDOverdriveCtrl in linux. Especially in a laptop, I think you're going to burn something up, like your cards. And yes, underclock memory to reduce temps. Too low will also reduce hashes so experiment or use other people's values. If I don't turn the fan speed up, my cards will rocket to high temps very fast. Yeah, I'm stressing out the fans more but I've also read on here that higher temps shorten the life of your chips (just as higher fan speeds shorten the fan life)