I managed to get both CPU and GPU client working, but the GPU client has to be console type (assigning different machine IDs didn't work). Now it seems both clients are working fine even with Catalyst 6.12.
The Windows GUI is quite slow with only the GPU client running, but gets noticeably faster with both clients enabled. Looks like the GPU needs to be fed by CPU and with both clients running the GPU has more time for graphics...
edit: I think I just found new hardware stress-testing tool - I have never seen such high temperatures on my system as while folding on CPU+GPU
