Yeah I think all the negative responses are because stability and power conservation are bigger issues right now, whereas there's more than enough performance for anything mildly sane you might want to do with an N900...that said, if you want to risk your unit, it looks like 800Mhz would be quite safe for the CPU since other devices with the same hardware are running this clock speed right out of the box. The only issue is if it might cause a problem with other hardware - these devices weren't made to take different hardware so you never know. I'd try increments of 10Mhz or so until you reach 800 stable if you want to go through with this - remember there may be no way to reset the clock speed. You can't reflash if it doesn't boot because the clock speed is too high.

Overclock with a low battery - this will make it glitch out more easily. That way when you plug it into an AC power source and the voltage increases a bit, it should make the CPU a little more stable (this is how it works in PC overclocking anyways) and give you a chance to reset the clock.

Edit: BTW disabling the CPU clock scaling is a very bad idea! You'd probably see zero performance increase, and it would eat through your battery like crazy and probably heat up the unit a bit. The CPU's clock speed will increase with demand. This is a good thing. No need to have it running at max speed all the time.