Any way to "choke" a process's CPU time?


Crotchety UI Nitpicker
I'm doing a lot of video encoding these days, which takes several hours, all the while using 90+% of my CPU. The problem is that this sends my fan whirring loudly for many hours, which is very annoying. I usually like to let the encoding go while I sleep, but the noise is just too much. It's also annoying in the day when I like to listen to music while using my computer.

Is there any way to tell a process that it can't use any more than, say, 50% of my CPU? That would probably cut back on the fan noise a bit, and I don't really mind if the encoding takes much longer (it'd probably finish overnight anyway).

I'm aware of renice'ing, but even at the maximum niceness level it still uses 90+% when no other apps need the CPU.
What would happen if you went into "Preferences - Energy Saver - Options" and set the Processor performance to "Reduced"?
I actually thought of that, but I don't see the option anywhere. The only options I get in that tab are auto-wake settings, "Allow power button to sleep the computer", and "Automatically restart after a power failure". I thought it used to be there, but I decided I was crazy and it was just a laptop thing. Am I supposed to have that option on my Mac mini?
Well I've definately got that option, I'm running tiger 10.4.2 on an iMac G5 and have my my performance set to "Highest".
I have the options of Highest, Reduced, and Automatic.

This option shows up right below the setting for "Automactically resart after power failure"
I don't think the Mac mini's G4 CPU supports speed throttling like the G5 processors do. It runs at one speed and one speed only, while the G5 processors (and some laptop G4 processors) can down-clock their speeds to conserve energy.