Hi Country Bumkin, I'm not sure if the procedure is any different when using Linux, but here is the third example which I forgot to post...

Third machine: Introducing GPU tasks to a machine doing CPU tasks exclusively. (therefore no apps_info.xml being used)

• First part of the procedure, create your apps_info.xml but do not deploy it.
  
• then goto yout BOINC preferences and enable the option to use GPU
  
• then watch and wait to see that at least one GPU task has been downloaded onto your machine. If it has already started, you have the option to let it finish or you can now move to the next step.
  
• SUSPEND all tasks and CLOSE BOINC
  
• now place your apps_info.xml into the projects folder (in windows this will be by default C:\ProgramData\BOINC\projects\www.worldcommunitygrid.org) now restart BOINC manager and you should see your chosen amount of GPUs being started.
  

In principle, there should be no difference when doing this in Linux, so try this and come back if there are any problems

Instead of app_info, you can use the new app_config.xml instead. It allows you to specify the number of GPU jobs you want to run on a given project (e.g., hcc1) without having to specify all the other CPU projects that you want allowed. And it can do other things, such as specify how many of a given CPU project are allowed to run at once (being especially useful for CEP2, which requires a lot of resources).
http://boinc.berkeley.edu/dev/forum_thread.php?id=8051&nowrap=true#46717

But to use it, you will need a recent alpha version of BOINC; 7.0.42 is the latest.
For Windows:
http://boinc.berkeley.edu/dl/boinc_7.0.42_windows_intelx86.exe
http://boinc.berkeley.edu/dl/boinc_7.0.42_windows_x86_64.exe

The full list is here:
http://boinc.berkeley.edu/dl/

I've just gotten used to app_info and it's working for me so I think I'll wait until the app_config matures (or app_info is banned).

You are welcome. But I use 7.0.42 on two machines, both with an HD 7770 running HCC with no problems. It is all experimental anyway at this point insofar as I can see.

The problem with formal releases is that it could be a long time before one comes along, and they are fixing numerous bugs in the meantime with each alpha release. Just be prepared to go back to an earlier release (of version 7) if that is necessary, but I have not had to do that for a long time now.

andzgrid,

No, I am running Win7 64-bit on both machines. I like to stick to one work unit per card, having sometimes had problems with more than one. With one of the earlier betas of the 12.11 drivers, I would get errors on the results (which did not show up on BoincTasks on my PC), and it would even crash the other PC (BSOD). I tried it again on beta 11 of the drivers on the first PC, and it worked OK that time, but don't want to risk the second one at all, since it has a Ramdisk on it that does not like crashes. But I am limited on CPU resources with an E8400 dual-core on that first PC, and didn't really want to devote two cores to it.

If you have more CPU power, I think it would be worthwhile to try though. But there are diminishing returns; it takes proportionally more CPU power than you get back in performance, at least on my PC, maybe due to I/O limitations somewhere in the system. On the HD 7770, I would not do more than two work units per card, even if I had more CPU power, since the power to the card and fan noise would increase too much, but you might want to try more.