Well I'd choose 2 sticks of 2GB DDR2-1066 (PC2-8500) over 1 stick of 4GB DDR3-800 (PC3-6400) any day, dual-channel or not.
Real world consumer applications / usage don't really gain any significant benefit from dual-channel, so I would look at the underlying speed of the RAM (and the max speed the mobo supports).
But at the same speed / transfer rate, you might as well go for 2 x DDR2, as the RAM is cheaper.
The fastest DDR3 is now twice as fast as the fastest DDR2, so in that situation, dual-channel benefits are utterly insignificant.
SD