I'd be cautious about limiting your CPU speed. On laptops that reduces performance and *reduces* battery life. For intel processors,
race to idle is your best bet, and I have no reason to suspect that the DroidX's processor is any different.
To put it in simple terms, modern processors use virtually no power while they're idle. They do use less power to run at 300 MHz compared to 1000 MHz, but the difference isn't proportional to clock speed. Generally it's something like 70% power usage for 300 MHz compared to 100% for 1000 MHz. So, if the processor might take 1 second to do something at 1000 MHz, which requires 100% power for 1 second. At 300 MHz it'd take 3 seconds to do the same thing, and use 70% power for 3 seconds, which is equivalent to 210% power for 1 second. Therefore, it uses twice as much power overall.
Just for fun, this also explains how a widget can use virtually no CPU but kill your battery life. Some widgets (e.g. live wallpaper) update their graphics several times per second. (Note that the graphics don't need to actually change, it might just redraw the same thing 30 times per second.) This will keep bringing the processor out of its power saving "idle" mode.