The real question is how you plan to fit four sodimms into the two available slots
Anyway, you can run dual channel if you use a matching pair, but running 1 GB dimm + 512 MB dimm will cancel out dual channel.
That being said, while dual channel is nice, it's not imperative.
I'd rather have 1.5GB single channel than 1 GB dual.
Also, adding another 512 MB isn't likely to work in dual channel anyway, unless you find the same sodimm.