In my opinion the increased latency is not worth it. It doesn't gain much.

About the highest tradeoff that can be worth it is 1866

I'd grab one of the center-of-the-bell-curve 1600/1866 RAM kits. Extra RAM speed only matters significantly if you are trying to max out IGP performance for some silly reason.

Spend money on a SSD, or a faster GPU if you are going to game.

you can't change the base clock which is 100mhz, but you can change the ratio of memory clock to base clock, which is in increment of 1.33*bclk. that is, you can set memory clock to 1600, 1866, 2133, 2400 and 2666.
while memory clock over 1600 has only marginal boost to overall system performance, i would say that the price of ddr3-2133 and 2400 dimms are also affordable, so i would like to pay some money for that little boost.

Btw dual channel gives important boost in performance?I'm thinking about buying 8g now and 8g later.

Yes dual channel is important - has been since P4. Especially if you're using an IGP, but anything memory intensive benefits from increased bandwidth.