EDIT: please do not install it yet. I found a possible issue.

I have uploaded a new version that is supposed to support 125,250,500,550,600,700,750,800,850,900,1000MHz and sets the default max to 600MHz.
for freq >= 850 the DSP is set to 500MHz.
You should be able to change the frequency range as root using
I currently don't have my USB cable (for reflashing) with me so I can't test it myself
If there are any volunteers please let me know whether it works.
Installing this new kernel is at your own risk, of course.
Be prepared to reflash the stock kernel per USB!

You can download version 19 from
http://maemory.com/N900/overclock/
install all packages and the kernel-flasher-maemo last.
Good luck!