Dual-channel mode doubles memory throughput, and is why relatively slow memory (e.g. DDR2-533) can keep up with a C2D's effective 1066MHz FSB. If your software app is limited by memory bandwidth, this will help a lot. If it isn't, it won't.
Typically this is most helpful in apps that process a lot of data into/out of memory (*not* the hard disk). Photoshop is one such app.