The dual channel will run faster but if you are loading up lots of memory intensive applications and maxing out the 1GB then you would be switching to virtual memory and all the benefits of the dual channel would be lost.
I had the option to get 1GB dual for my macbook or 1.25GB single channel and I went the dual route because the speed in everyday use is more important for me.
rather than have 2 x 512 meg ram running dual channel rather put 1 gig in for the moment till you have the cash for another 1 gig ram and then u run dual channel
if you ever want to upgrade to 2 gig ram you will need to buy another 2 x 1 gig sticks