Well, using the FTDI chip for an USB<>RS232 interface was a great idea. But still no respons from the dosonchip module. >:( Connecting the RX and TX did work though, so the trick is okay.
So, back to the original question. Does anyone know easy to use modules to interface between arduino and SD/CF card or USB stick? The vinculum VDIP1 seems okay. But I haven't found examples yet how to use it.