Windows 7 - Will dual channel memory noticeably improve performance?

I have a dual-core laptop with 3 gigs of ram running Windows 7x86. If I understand correctly it's not running in dual channel mode because of the size mismatch. I can replace the 1gig stick with another 2gig stick which should enable dual channel mode, but will it make much difference? Is there a synthetic benchmark I can run to test the bandwidth? The extra memory isn't expensive and I would get a little more usable ram, but I'm guessing the benefits will not be dramatic. Any thoughts?

I think you are guessing right. A lot will depend on the RAM usage pattern. You may get a small hit for the caching of Superfetch, but for running your apps you will most likely stay below the 2GB mark anyhow. That is easy to check in Resource Monitor > Memory tab.

Use CPUz(look on the memory tab) to see if in fact it running in single channel mode, it most likely is running dual channel, laptops as well as most newer desktop boards will run sticks of different capacities in dual channel mode.

CPUID - System & hardware benchmark, monitoring, reporting