Hi,
my problem has gone. Today I contacted Asrock Support and asked if they
know the problem and if a BIOS update could help.
They sent me the newest BIOS version 3.21A in a special version. My
BIOS boots normally and is showing 3.328 MB RAM (4GB installed) in
DualChannel-Mode.
After logging in to Vista the system shows normally 4GB. It seems not
to be a mainboard problem. Asrock support wrote that the chipset on this
board has problems with 4GB RAM. After installing the new BIOS all
really works fine!

Hope this could help others too.

Greetings from Austria,

Thomas


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VitoDonCorleone