"My understanding was that a 32 bit machine could address up to 4 Gig of Ram."
its not the machine thats the problem: WINDOWS has an artificial restriction built in. My understanding is that if you use 32-bit Windows Ultimate, the restriction goes away. It was partly a marketing restriction by M$ to differentiate between the various Windows SKUs.
But as I said earlier - its a funny situation with some machines reporting the full value.
As to single / dual channel: for most uses you won't see any difference in performance. You have to be really hammering the machine for it to show