2 platters means it has 30GB/platter density, which is pretty good, but it only helps for transfer rates. 5400rpm drives don't have as good access times as 7200rpm drives. If it's for data/mp3, 5400rpm is fine, but I wouldn't get anything else than 7200rpm for the OS since the price difference is so small.
I also agree that Maxtor drives are better than WD.