ok this is the way i have overclocked my n900.
install enhanced linux kernel for power users(settings) be careful while selecting this
and QCPUFreq from the app manager !

after installation open QCPUFreq and go the settings in the upper tab and select all the options (not sure about the last one , but i have checked it , dont know why :P )
and after saving it will give a pop regarding warranty .
after that it will take you back on the QCPUFreq main screen , and at the bottom you will see a slider , just move the slider forward to choose the suitable frequency .
i will suggest as many others suggested me to choose the ideal from the load presest ( in uppar tab same where you find the settings)
and there you go you n900 is overclocked