I finally installed more RAM into my PowerBook this morning, going from 512MB to 768MB. I can't say that it's made a huge difference, but I haven't had the computer grind to a stop yet today, either. Since it's been slowing down every afternoon for weeks, and I've been killing off Mail and NetNewsWire just to keep the computer usable with 512MB, this seems like progress to me.
I ordered it from TransIntl; they had the best price. It arrived last week while I was out sick, so I can't really comment on speed-of-delivery, but it was well-packaged, cheap, and works right.