I am planning to get a PC with 2 GPUs inside; so far I have never had any multi-GPU configurations, and hence no experience with this.
What I gather so far is that there are mainboards with SLI and 2-way SLI. Whatever the difference is.
Basically, my question is: does GPUGRID crunching work on multi-GPU-systems, so that the power and capabilities of both graphic cards can be used for GPUGRID?
Does SLI / 2-way SLI have any Impact for GPUGRID?

I am planning to get a PC with 2 GPUs inside; so far I have never had any multi-GPU configurations, and hence no experience with this.
What I gather so far is that there are mainboards with SLI and 2-way SLI. Whatever the difference is.
Basically, my question is: does GPUGRID crunching work on multi-GPU-systems, so that the power and capabilities of both graphic cards can be used for GPUGRID?
Does SLI / 2-way SLI have any Impact for GPUGRID?

No SLI nor 2 way SLI does not help at any Boinc project, it does help alot if you are a gamer though. Boinc is not currently designed to share gpu's the way games can. There are some projects that can share cpu cores, so maybe in the future gpu's could be tried, but it would have to make ALOT of difference in the crunching to the project, not just faster speed for us users.

No SLI nor 2 way SLI does not help at any Boinc project, it does help alot if you are a gamer though. Boinc is not currently designed to share gpu's the way games can. There are some projects that can share cpu cores, so maybe in the future gpu's could be tried, but it would have to make ALOT of difference in the crunching to the project, not just faster speed for us users.

thanks a lot for your quick reply.
Do I understand correctly though that if I buy a PC with one graphic Card, and than I simply put in another one on any other free PCIe slot, both graphic cards would then do their own work? So it would make sense to have more than one card for GPUGRID crunching?

Do I understand correctly though that if I buy a PC with one graphic Card, and than I simply put in another one on any other free PCIe slot, both graphic cards would then do their own work?

Yes.

So it would make sense to have more than one card for GPUGRID crunching?

Yes.
If you have two different GPUs, BOINC will use the faster one by default, and you have to configure the BOINC manager through the <use_all_gpus> setting in the cc_config.xml file.
When you plan to use multiple GPUs in the same system, you should consider also the cooling of the cards, as the lower one will heat up the upper one(s), especially those cards which have custom cooling with axial fans.