Well, it is clear that a dedicated GPU will outperform Intels IGPs.
The question is rather whether usage of an OpenCL-capable IGP can compete with the usage of a CPU core of that same system (of which one usually has to be kept free to "fire" the GPU).

Michael.

P.S.: In my hands, so far neither an HD 4000 nor an HD 5500 IGP running at full load with SETI will cause any lags or crashes even when working regularly on that same machine (browsing, office, etc.).

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Well, it is clear that a dedicated GPU will outperform Intels IGPs.
The question is rather whether usage of an OpenCL-capable IGP can compete with the usage of a CPU core (of which one usually has to be kept free to "fire" the GPU).

It depends very much on the core, and the iGPU. Generally it can outperform a single core, but not multiple cores.
And one of the biggest issues is power/heat. The iGPU shares it's power & heat limits with the CPU- the end result is you can run the iGPU flat out, but loose the output of the CPU cores to do so; and without those cores you generally end up with less work being done than just running the CPU cores alone & no IGPU crunching.
Add to that Intel's rather poor (at best) support for OpenCL, so at this point in time iGPU crunching really isn't worth it IMHO.

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Grant
Darwin NT

Well, it is clear that a dedicated GPU will outperform Intels IGPs.
The question is rather whether usage of an OpenCL-capable IGP can compete with the usage of a CPU core of that same system (of which one usually has to be kept free to "fire" the GPU).

Michael.

P.S.: In my hands, so far neither an HD 4000 nor an HD 5500 IGP running at full load with SETI will cause any lags or crashes even when working regularly on that same machine (browsing, office, etc.).

It's complex topic. And strong dependence from iGPU generation here.
For introduction look this experiment: http://lunatics.kwsn.info/index.php/topic,1735.0.html
Worth to repeat it for other iGPU families.

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