I just replaced the fan and cooler on my XFX 6850 black edition and thought this would be a good place to share my experiences for others. The fan on my card got very noisy and made unhappy grinding sounds on occasion. I attempted a fix that I will link below, but with unsatisfactory results. In the end I was tired of the whine of the OEM cooler and decided to replace it all together.
First the normal disclaimer: This will void any warranty on your videocard. XFX reportedly has a very good warranty replacement program, so that's the best way to go if you have problems with your fan.
If your fan is grinding, the best thing to do first is to try and re-lubcirate the fan bearing. Here's a great thread on this forum which describes the process for this and other sealed XFX fans:
Unfortunately, this didn't fix my issue so I had to look elsewhere. I settled on the Arctic Cooling S1 Plus cooler for 2 reasons: 1 - I've used their coolers in the past to great success, 2 - My preferred store for cooling components had it for 25% off.
Manufacturer link: http://www.arctic.ac/us_en/products/cooling/vga/accelero-s1-plus.html
Before embarking on installing this cooler, heed my warning: IT WILL TAKE OVER 6 HOURS TO INSTALL. YES, OVER 6 HOURS.
Removing the stock cooler is a simple matter, for my xfx card it was 4 screws on the back of the card under the retention plate and 1 screw on the back-panel pci-covers that secure the plastic shroud. Do your standard TIM removal on the GPU core, and remove the VRM cooler by un-screwing the two screws on the back of the card. Finally, clean off the VRMs in much the same way. Tada. No, you can't keep the old VRM cooler, it's too tall.
The cooler comes with a thermal epoxy that needs to be mixed. The directions plainly say mix the two packets of goop together for 5 minutes, then install the new VRM and RAM heatsinks in about 15 minutes.
Here's the part that sucks: The epoxy will take 6 hours to cure. You have to leave the card alone while the glue sets. My suggestion is to start the install before going to bed and then complete it the next day. I let mine sit for about 11 hours (after 4 hours the glue was still wet). My understanding is that the newer version of this cooler and other AC coolers with epoxy have a new mixture that should only take an hour or two to cure.
Once that's cured, installing the cooler itself will only take 3-4 minutes. Follow the directions to install some 3.5mm spacers onto the cooler, place the videocard onto the cooler, and screw down the 4 screws. Incredibly easy, just follow the directions.
First, you CANNOT run this passively. The VRMs and GPU will get too hot under load. I had two fairly quiet 120mm fans which I attached with some double-sided foam tape directly to the cooler. They sit at weird angles, but it gets the job done.
Running Furmark burn-in with two fans led to a temperature of 59-60ºC at a noise level that was imperceptible above the other fans in my case. Quite the departure from the hairdryer noise from before.
TL;DR: Arctic Cooling S1 plus works but needs one or two 120mm fans. Installation takes ages for glue to cure. Results are 'happy smile face'™ in temp and noise.