10.8.3 seems to be treating me right. I've got 20" early 2008 with Mountain Lion, which I installed a few weeks ago. It's been freezing as described (movable mouse, but nothing else) at least once a day. I was all set to fall back to the earlier kexts yesterday, then saw that it wanted to do the upgrade to 10.8.3, so I let that happen. Before swapping out the kext files I checked to see if they were updated. They were. Decided to leave it alone to see if there was any improvement. My system has been running smoothly since, and even seems faster. yay!
>find /System/Library/Extensions/ATIFrameBuffer.kext/ -type f | xargs ls -l
-rw-r--r-- 1 root wheel 2354 Mar 10 21:38 /System/Library/Extensions/ATIFrameBuffer.kext//Contents/Info.plist
-rwxr-xr-x 1 root wheel 147280 Mar 11 17:20 /System/Library/Extensions/ATIFrameBuffer.kext//Contents/MacOS/AMDFramebuffer
-rw-r--r-- 1 root wheel 158 Mar 10 21:42 /System/Library/Extensions/ATIFrameBuffer.kext//Contents/_CodeSignature/CodeDir ectory
-rw-r--r-- 1 root wheel 80 Sep 10 2012 /System/Library/Extensions/ATIFrameBuffer.kext//Contents/_CodeSignature/CodeReq uirements
-rw-r--r-- 1 root wheel 1745 Mar 10 21:42 /System/Library/Extensions/ATIFrameBuffer.kext//Contents/_CodeSignature/CodeRes ources
-rw-r--r-- 1 root wheel 4064 Mar 10 21:42 /System/Library/Extensions/ATIFrameBuffer.kext//Contents/_CodeSignature/CodeSig nature
-rw-r--r-- 1 root wheel 466 Mar 10 21:41 /System/Library/Extensions/ATIFrameBuffer.kext//Contents/version.plist
>find /System/Library/Extensions/ATISupport.kext/ -type f | xargs ls -l
-rw-r--r-- 1 root wheel 2421 Mar 10 21:38 /System/Library/Extensions/ATISupport.kext//Contents/Info.plist
-rwxr-xr-x 1 root wheel 500384 Mar 10 21:38 /System/Library/Extensions/ATISupport.kext//Contents/MacOS/AMDSupport
-rw-r--r-- 1 root wheel 154 Mar 10 21:42 /System/Library/Extensions/ATISupport.kext//Contents/_CodeSignature/CodeDirecto ry
-rw-r--r-- 1 root wheel 76 Sep 10 2012 /System/Library/Extensions/ATISupport.kext//Contents/_CodeSignature/CodeRequire ments
-rw-r--r-- 1 root wheel 1745 Mar 10 21:42 /System/Library/Extensions/ATISupport.kext//Contents/_CodeSignature/CodeResourc es
-rw-r--r-- 1 root wheel 4064 Mar 10 21:42 /System/Library/Extensions/ATISupport.kext//Contents/_CodeSignature/CodeSignatu re
-rw-r--r-- 1 root wheel 466 Mar 10 21:41 /System/Library/Extensions/ATISupport.kext//Contents/version.plist
find /System/Library/Extensions/ATI2600Controller.kext -type f | xargs ls -l
-rw-r--r-- 1 root wheel 4976 Mar 10 21:38 /System/Library/Extensions/ATI2600Controller.kext/Contents/Info.plist
-rwxr-xr-x 1 root wheel 1856344 Mar 11 17:24 /System/Library/Extensions/ATI2600Controller.kext/Contents/MacOS/AMD2600Control ler
-rw-r--r-- 1 root wheel 161 Mar 10 21:42 /System/Library/Extensions/ATI2600Controller.kext/Contents/_CodeSignature/CodeD irectory
-rw-r--r-- 1 root wheel 80 Sep 10 2012 /System/Library/Extensions/ATI2600Controller.kext/Contents/_CodeSignature/CodeR equirements
-rw-r--r-- 1 root wheel 1745 Mar 10 21:42 /System/Library/Extensions/ATI2600Controller.kext/Contents/_CodeSignature/CodeR esources
-rw-r--r-- 1 root wheel 4064 Mar 10 21:42 /System/Library/Extensions/ATI2600Controller.kext/Contents/_CodeSignature/CodeS ignature
-rw-r--r-- 1 root wheel 466 Mar 10 21:41 /System/Library/Extensions/ATI2600Controller.kext/Contents/version.plist