I've got a Sapphire HD 4850 myself. Very impressed. The fan is variable and at idle it's nice and quiet. It does however, like all 4850s, get very hot. It idle's at around 70°C and at maximum stock clock speeds you'll see temperatures of 90°C to 100°C. While the fan is quiet, many users have complained about the fan speed controller not forcing enough RPM when running at full clock speeds. Many manufacturers (not just Sapphire) have seemingly chosen noise as a priority.
There is a workaround to this for those who are worried, and for those who want to attempt overclocking with the stock cooler. It involves tinkering with a file in notepad and is very straightforward having tried it out myself. Basically, you can override the automatic fan speed algorithm and set your own target speed for the fan, from 0% to 100% (which is extremely loud!). Instructions can be found here: Radeon HD 4850 Heat issues - Guru3D.com Forums
Having said all this, these temperatures are the operating norm for the 4850. The single slot cooler doesn't do much favours either, but overall, my system still runs fairly cool and the 4850 performs perfectly, untweaked.
There are of course, other good graphics card manufacturers around too.