Yes, they key is the notches. SDR SDRAM has two notches, DDR SDRAM has only one. This physically prevents the insertion of the wrong RAM type in the wrong slot, and the retention arms will not close.

If I had to guess at your probelm attempting to run those old modules with the news ones, it would be that your 256 Module runs CAS 3 at 100 MHz, while the old ones run CAS 2 at 100 MHz. If the BIOS isn't smart, it may be trying to run your new module at CAS 2 to match the others. There may be an option in your BIOS that enables you to force CAS 2.