Well to answer your main question. Dual channel memory is not a special type of memory. It is simply when you use two DIMMs of the same size and speed and place them in the correct slots on your mother board used for dual channel memory. Each DIMM will be a single channel, the pairing of the two channels is done at the memory controller level, it is not a feature of the DIMM itself. There is nothing different about buying two single DDR2 DIMMs and a "dual channel" kit with two DIMMs in the kit. Ideally you should only use DIMMs from the same manufacturer (at least the same chips being used if you mix brands) but this is just to help eliminate possible stability issues, this does not mean you can't mix and match brands but YMMV as to stability at stock clocks and/or during overclocking.
Now about your 2.8GB's of RAM being seen. If your signature is correct and you are using Vista 32-bit and 8GB's of RAM, then there is your problem. You need to have a 64-bit OS to use more than 4GB's of RAM. I'm not sure why it is reporting such a low number but I would blame the OS first and not conclude it is faulty hardware.