Added to my last post, I've also found that the memory that came with the machine is 1.5 volt. crucial.com are listing their 2 x 4 GB 1.35 volt boards as compatible, which may offer a slight power saving improvement if you upgrade the memory.
[Please note: you can only use more than 4 GB memory if you changed to the 64 bit version of Windows 7 (requires a new windows install - see posts in pages 3-4 of this thread for details). The 32 bit version, that comes as standard with the DM1-4xxx notebooks, cannot use more than the 4 GB that the DM1 ships with.]