Laptop Memory: Difference between DDR2s?

[dmw16]

I am trying to upgrade my GF's laptop memory. We bought DDR2 memory that is 667mhz (her's is 533mhz), but I assume it is backwards compatible. We installed and no dice. The package said "For Dual Channel Systems". Are all DDR2 systems dual channel? Do some brands not underclock? The laptop is a gateway if that makes a difference (model MX3220)

thanks.