I have two iMacs: a late 2009 27" iMac with an i7 and a mid 2010 21.5" iMac with an i3. They both report sse4_1 and sse4_2 in Boinc. Both computers get various versions of the apps. They both get SETI@home v7 7.00. I've also seen 27" iMac running ssse3 and sse41 apps. I've not seen the ssse3 app on the 21.5" iMac. Each iMac will run multiple apps on different threads. Currently my 27"iMac is running a few threads of the non-optimized 7.00 app, a few ssse3 threads, and one sse41 thread. Is there any way to tell Boinc or SETI@home to just use the sse41 app on these computers?

That's what the app_info.xml does. Instead of allowing the Server to test and guess which App to send your machine, You list the Apps You want to use and tell the Server which tasks to send. Basically that's how your HD4670 machine is now setup. You are telling the Server You are going to run the SSE41 & ATIr2934 Apps and you need it to send you Multibeam tasks for those Apps. If you used the same app_info.xml on the machine with the HD4850 the Server would Stop sending you those ATI HD5 Apps that will not run on a HD4 card.

The trick is to make sure the MultiBeam_Kernels_r293x.cl_RadeonHD4xxx.bin_v7_xxx file is Not in the setiathome.berkeley.edu folder when you request new tasks the first time. That way the first downloaded ATI task will Run instead of producing an Error and give you Time to Suspend the other waiting ATI tasks. Once the Running ATI task is finished go back to the setiathome.berkeley.edu folder, remove the MultiBeam_Kernels_r293x.cl_RadeonHD4xxx.bin_v7_xxx file again, and Resume another ATI task. Once the ATI task is running you can temporarily resume the other ATI tasks long enough to Download new tasks when needed. Afterwards you should Suspend the waiting ATI tasks so one doesn't try to start while the MultiBeam_Kernels_r293x.cl_RadeonHD4xxx.bin_v7_xxx file is present.

Hopefully Urs will have a fix before long...