Sometimes.......Good Things Happen.....Or Do They?
Dual channel operation is sometimes possible with RAM running at different speeds. But, it does depend on the chipset and board combination, in conjunction with the type of RAM involved.
All of the DIMMs would need to be "generic" types, in other words, conform to basic established standards, which in the case of DDR2, is 1.8V and at or about 5-5-5-15 timings.
This goes only to the ability to have it work. I have one desktop, (which I'm using at the moment), that a one point had RAM of 3 different brands, running at 2 different speeds, yet still returned system information of "dual channel operation".
Disclaimer, this is DDR I'm speaking of, and the speed of operation was the slower of the 2 speeds involved. (The faster DIMMs are always clocked down to the speed of the slowest sticks).
As to whether 6GB is better than 4GB, I have no opinion. I hold the odd belief that adding more RAM doesn't always obtain the desired, (or fantasized about), results. There are other things that keep popping into my head such as, "don't you, at some point, run out of CPU clock cycles"? But, when I was younger my father told me I think too much, so that could be what's happening here.