With regards to the CPU speed, the OMAP3 in the Pandora has been known to happily overclock to 900mhz (cpufreq support for the OMAP3 is in kernel mainline) so with any luck decent overclocks may be possible on the N900 too. One thing to bear in mind though is that the CPU on the N900 has no heatsink, the case has no air vents and you really don't want to fry your £500 phone!!