Yeah, I sorted out, or better I have been let to this link:
which describes the problem with LowMem and HighMem when having 1024MB of RAM.
I could have sworn that I had the same amount of memory already before BadRam installation. I compiled the kernel with HighMem support, now I have the full amount. However In the article they recommend not to enable HighMem on just 1024MB, because the mapping seems to be too intense. Rather, one should be happy, without the 128MB en plus.
Anyway, thanks for your help.