Okay, according to Activity Monitor, when I'm slamming it, Lightroom is using 600-700MB of real memory and up to 1.7GB of virtual memory. Watching a rendering task run, I got to watch my free memory quickly dive to zero, then back up again, and dive again. Looks like Lightroom was pushing other apps out of real memory. Both cores were slammed at 100% the whole time.

If I'm reading the tea leaves correctly, Lightroom would definitely be happier with more memory, but it's got enough going on to take advantage of more CPU power if I had it. Part of the issue seems to be slow memory leaks, since it runs much better when you've freshly started.