Performance memory makers such as Corsair make "paired" memories (Twin pack) just for this purpose. These memories are "tested" to work together in dual channel memory system such as those with nforce2 mobo chipset.
If the paired memories are not of the same size and brand and model, you can run into serious instability issues.
At the least make sure that you get the same size, brand and model no.
Keep in mind though that performance increase from dual channel is not that great. My 3dmark 2001 score only went up 500 points when I went to dual (1 gig) from single (512mb).