Thanks BALLOON
it works with some help. Installing opera_12.16.1860_i386.deb via GDebi fails because of missing packages. I had to install it manually using:
sudo dpkg -i opera_12.16.1860_i386.deband then
sudo apt-get -f installand now it works!
BTW
My problem was, that I expected my machine to be x86 architecture. I'm used to Windows naming convention and Windows XP originally installed on my netbook was x86 (32-bit in MS language). Acronym of i386 in MS language refers to old Pentium processors.
You are right, I kept downloading amd64 (opera_12.16.1860_amd64.deb) file.
What is the meaning of x86 and i386 in Linux world?