I got a lovely 3870 a few months ago complete with stock heatsink / fan, which did the job but was pretty noisey given than everything else in my system is properly quiet.
If anyone tells you that the stock cooler is quiet, they don't have a 'quiet' system as i'd define it!
Swapping the the stock cooler for the AC-S1 (
£15 from Scan) took about half an hour, trying to dig out the right sort of mini-screwdriver and going through a nervous few minutes pressing down really hard trying to screw the new heatsink in (and being worried about crushing the GPU core chip), but got there in the end. After a couple of false start reboots (blank screens, i was full of regret thinking i'd borked the whole job) it all worked well.
I now have again a properly quiet system, and putting my hand on the metal fins seems to indicate a much cooler temperature than i'd have imagined. My P180 case does seem to do a great job with airflow. I'm not so interested in overclocking the graphics card as it does what i want, but it's now quiet and stable, so i'm happy.