I was going to say it doesn't make much difference, but I did some research and it turns out the old Core Duo (not Core 2 Duo) MacBooks are not only incapable of recognizing more than 2 GB of RAM, but they actually won't boot with more than 2 GB installed.
So if you do buy a 2 GB module, you'll have to ditch both your 512 MB modules anyway. In that case you may as well go with a matched pair of 1 GB modules since you get the added benefit of dual channel RAM.
The benchmark tests I've seen show almost no measurable difference between single- and dual-channel RAM (a few % at most), but the point is it can't hurt. The cost is about the same ($50-$60 either way) so there's no reason to get a single 2 GB module.