Thanks for the hint! I'm looking at this paraghraph:
https://leste.maemo.org/Nokia_N900#Overclocking

"Overclocking is now available in -devel using cpufreq boost.
It uses 250-500-550-600-720-805-850MHz frequencies
To enable it, edit:
/sys/devices/system/cpu/cpufreq/boost
And replace 0 by 1."

But this here gives me a headache - /sys/devices/system/cpu/cpufreq/boost doesn't exist on my machine. How do I get it?