Yes you can run dual channel with another stick of 512MB. You dont have to enable dual-channel, just make sure you put the memory modules in the right slots (typically slots 1-3 or 2-4 or both). The memory slots should have two differnt colors to represent the two memory channels.
Keep in mind that your system will run the PC-3200 module at PC-2100 speed.
It's more important to get memory with the same timings and chip configuration (i.e. #chips/side) rather than brand. If you can, it's better to match brands too.