When I started mining, I just went with the newest Catalyst (I'm now at 11.5) and newest SDK. This seemed to work great for me, have been getting 214Mh/s with my HD5770 OC@965Mhz and 300MHz RAM.
However, my curiosity has gotten the best of me, and I've been trying to install the ATI Stream SDK 2.1.
Problem is, even though i completely remove all SDK 2.4 related stuff, the SDK 2.1 installer installs everything and gives me a green OK at the end, but doesn't actually enable OpenCL functionality on my machine. 2.2, 2.3 and 2.4 are all able to install just fine and work with my HD5770, but 2.1 refuses to enable OpenCL.
Windows 7 x64 (Ultimate SP1)
XFX Radeon HD5770