OK, so I can buy it then with just the 1 GB base, then upgrade it to 3GB with memory purchased elsewhere (I see that OWC is already offering a 4GB upgrade for this MacBook for $159). I was confused as we have an iBook, and we can only take out one of the RAM inserts, I believe. So this isn't an issue with MacBooks?
Correct. The MacBook has 2 RAM sockets, both user accessible. To get to 3 Gb you'll remove your 2 x 512 and install 1 x 1 Gb and 1 x 2 Gb. The 2.5 Gb configuration is workable too, just add a 2 Gb SODIMM to the existing 512 Mb module. The RAM is the same as for the MacBook Pro SantaRosa, so there is no magic to it.
The iBook is a different design with some RAM soldered on, and only one socket.