I've been playing around trying to optimise my system temperature on a dual G4 that runs all the time. I've installed Apple's CHUD version 3.5.2 (FTP site for CHUD tools) to access Nap mode -- I don't think newer versions support Nap mode.
Apart from anything, though, I hate the noise the machine makes with nap mode on -- there's a noticeable change in frequency as the CPU enters and exits Nap mode. So I wrote an AppleScript (adapted from the script in this hint) to set nap mode and single processor options to reduce power consumption when the machine's not in use, and restore them when you come back. Comment out the cpu_count line if you're on a single processor machine.
Save the file as an application, with Startup Window unchecked (it took me a while to figure that one out), and add it to your login items -- then you're all set.
[robg adds: I haven't tested this one.]
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Mac OS X Hints
http://hints.macworld.com/article.php?story=20070406120941154