Dual-channel Memory Options

I have an Inspiron 1440 (Best Buy model) with 3GB RAM (1x1GB+1x2GB).  CPU-Z reports that my memory configuration is dual-channel.  I was under the impression that in order to have dual-channel, both modules needed to be the same density (capacity).  Is this correct or is CPU-Z correct?

I'm considering buying a matched pair of 2GB SODIMMS (PC2-6400) to give me a total of 4GB.  Do you think this would give me much of a performance boost?  The computer came with Windows 7 Home Premium 64-bit.

RAM is kind of expensive right now and since this is a budget laptop, I'm not sure if the performance gain would be worth the cost.  I could save money by buying a single 2GB module and replacing the 1GB SODIMM with it.  But then they wouldn't be a matched pair.  As long as both modules are both PC2-6400 and have the same latencies (timings), then I should get dual-channel mode, right?  Even though they would probably be different manufacturers?