What is required to use DDR2 DualChannel im memory modules ? How much more speed can I expt?

I am planning to upgrade/change my notebook to new DDR2 SO-DIMM RAM modules.

I have heard that DualChannel capable memory modules could speed up the computer significantly.

Is this true?
How much speed-up can I expect ?

AFAIK the two modules must fit together in order to be able to use CualChannel.

What means "fit"?

Do they have to be the same capacity (e.g. both 1 GB or is a 512 + a 1024 GB combination possible as well?)

Do the two RAM modules have to have ECC feature?

Do the two RAM modules have to have both the same FSB (or is e.g. one 400 Mhz and the other 533Mhz possible) ?

Do the two RAM modules have to be both double sided or single sided ?

Jason