Well, I know that nice is not what you need to use. Nice prioritizes existing processes, however I don't think it is capable of throttling CPU usage. Essentially if you have a single low priority process running, it would get 100% of the CPU time, just because there is no competition.
I've never done it, but from doing a little googling, I've seen references to using ACPI to throttle back the CPU speed. I found this site
which explains it in a bit more detail.