What processor does your system use? If you have an Athlon XP processor then don't bother abour dual channel memory because it won't do anything for your system.
You do NOT need to have special modules for dual channel configuration. It would be advisable, however, to have the second module identical to the original module. It should really be the same make/model.
Sometimes, mixing RAM modules from different manufacturers can cause problems even in standard configuration. This is especially true when cheap, generic (unbranded) RAM is used. I'd personally never 'mix and match' different RAM modules. I always think it best to sell off the original module and purchase two matched modules for the upgrade.