After swapping-around things that seems to have solved the crashes and finally having time to install a different OS is up and running on the Ati-5850 again. For the moment under win8-preview, so many things probably won't work, but atleast v7.0.28 BOINC finally installed without any problems.

Configured Einstein@home to run 2 tasks at once on the GPU, by setting "GPU utilization factor of BRP apps" to 0.5 in the Einstein-preferences. On client this equals 0.5 GPU, 0.5 CPU per task.

Started-up GPU-Z, verified OpenCL was installed, and started downloading Einstein-GPU-work. Then the 1st. task started it according to GPU-Z used roughly zero of the GPU, and both "Elapsed" and "Remaining" was counting-up.

Then the 2nd. task started on the other hand, GPU-usage jumped to roughly 75%, and one of the cpu-only tasks was paused. While "Elapsed" for both GPU-tasks counted up, "Remaining" now started counting-down, but granted not as fast as estimated.

Both finished after roughly 2h10m, meaning on average 65 minutes per task. Looking on actual cpu-time, this was roughy 30 minutes/task, so by running 2 at once this means 1h cpu-time for 2h10m run-time or Einstein@home uses 45% of a cpu-core.

Now no idea if so short run-time was just a fluke and neither if my GPU just computed garbage and will fail validation, but atleast these results indicates Ati is much faster than either spingadus or kmanley57 results indicated, as long as allows Ati the usage of a cpu-core.