Wie kann man auswählen welche Grafikkarte benutzt werden soll... leider nimmt das Programm immer die schwächere Karte.

Make a cc_config.xml file in your BOINC data directory (if there is none yet).
Add into it:

<cc_config>
<log_flags>
</log_flags>
<options>
<use_all_gpus>1</use_all_gpus>
</options>
</cc_config>

Save changes, then exit BOINC & restart it. Since GPU detection decisions are only done at BOINC startup, re-reading the cc_config.xml file will not do anything here on a working BOINC. Hence the need to restart.

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Muss ich Löschen die > < ?

I make the datei, in den Haupt ordner ?

Muss ich Löschen die > < ?

You can just copy the lines as they are, with the < and > signs and put them into Notepad. There's no need for a special XML program.

When done, do File->Save As...
File name: cc_config.xml
Save as type: All files
Format: ANSI
Path: normally into C:\Programdata\BOINC\, but that depends on your installation and maybe if the German version of Windows changes the directory names. Then it's probably C:\Programdatei\BOINC\

But you can check that in your BOINC start-up messages.

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