When I posted the above message the load on the machine spiked from 1.21 to 1.61 and there were 111 connections to port 80, about half of which looked like they belonged to a search engine. As I'm looking at it now, the load is stuck above 1.00 and there are 5 zombie processes. It's using 722 MB of memory.
The machine averages just over 256 Mbit/sec of bandwidth per month -- about 70 GB, but that includes the nightly backups, too. It's busy, but not excessively so.