Don't use 2.2, bugs and low performance. SDK 2.1 or even 2.4 would be better. I have 3 Xtreme 5850s and they all mine at around 358~ with a modest overclock (900 core, 300 mem)
 
Here's what you want to do:

1. Uninstall all ATI drivers and software through control panel
2. Run this script to clean up the rest, Genetix Cleaner v4.85 - http://www.mediafire.com/?1dkuw184mdzc65d
3. Download and install Xtreme-G 10.10 Win7 32-64bit - http://www.mediafire.com/?ccl57w7l3wf4m8x
4. Download and install ATI Stream 2.1 SDK - http://developer.amd.com/Downloads/ati-stream-sdk-v2.1-vista-win7-64.exe
5. Download and install Sapphire TRIXX - http://us.sapphiretech.com/archive/gm/drivers/SAPPHIRE_TriXX_Installer_v4.0_634402083351463953.1.exe
6. Set core to 900, memory to 1200 (300x4), and voltage to 1100
OPTIONAL: Set fan speed to 100%

Enjoy high GHashes at 50°C~!