The 600M has a plastic case, and usually ships with lower memory, 4200RPM HDs, and a 64MB Graphics card. This is their home notebook line.
The D600 has a magnesium alloy case, has greater memory, 5200RPM HDs, but a 32MB Graphics card. This is their business notebook line.
They all have wireless abilites. The D600 model is more solid in my mind.