Don't fix your cpu at 900 with limits of 900 900, it will overheat potentially.

Moving the lower limit to 500 is a great idea though.

What power voltage levels do you have set? the kernel-config show command will show you.

I'm running:

FREQS="250:30,180 500:30,180 600:38,400 750:45,500 950:54,500 1000:60,500 1100:72,520 1150:72,520"

therefore voltage of 30 for 500mhz

If you're after speed, load swappolube and apply its proposed settings, even consider setting swappiness to 10.

Make sure you're running the PR1.3 firmware and ifs its a used n900 reflash all of it, you don't know whats been messed with.

Transitions is another area to look at, the n900 naturally 'wastes' cpu doing flashy zoom and fade effects you don't need.

not sure what current situation is with tweaking transitions but its worth doing.

merry christmas

cheers,
Dave