Have you looked at Arctic Cooling? They offer some awesome air-cooling solutions, and more often than not they are whisper quiet & for a low price too. They're all I've used on my CPU (and one GPU - Radeon 9800 Pro) for years and they're just awesome.
I'm using the Rev 13 cooler on my i7 and boy, it's massive... Almost grandiose in size, and it only cost about £15-£20, and I can't grumble at that price. Stays around 50-60C under load. Not too bad for an air-cooling solution.
Here's one cooler for the 5850:
Arctic Cooling Accelero TWIN TURBO II
That is the 'Twin' of the single version.
Actually, after some more investigation, their 'flagship' cooler is also compatible:
Accelero XTREME Plus II
It sports 3 fans.. More than enough fans for a graphics card! Haha.
You just have to check the dimensions (more namely - length), if you're using a custom PCB of the card vs the cooler. I'm assuming you're using a vanilla card.
I'm thinking that the XTREME Plus II card is so long, that it will fit anything... Or even overhang.. :/
All I can say is that the ASUS HD6xxx DirectCU II cooler is almost rudely loud when it's hitting 60%+. But it does an insanely good job. I have my card heavily OC'd and it
really needs hefty cooling :/
FYI, I <3 Arctic Cooling. Their prices are fair and their products are exemplary.