Big margin. 600Mhz > 800Mhz = 33% increase.

What would run hotter at such utilization?
66% @ 800Mhz or 100% @ 600Mhz

If it can handle 800Mhz, it might even run cooler.
All depends on the sample quality in your N900.

Overclocking does not mean we will constantly use that faster frequency at 100%.
It's the "instant" cpu power that it lacks sometimes.

When it shoots to 100% @ 600Mhz which it does frequently during certain operations, it's hitting a bottleneck.
Instead, if the cpu was capable of 800Mhz for its ceiling.
Then maybe it might only neded 80% at 800Mhz for that "instant".

Why do we have quad cores in our desktop pc's when 90% of the users will never really get it to use 100% on all 4 cores?
The more speed headroom you have, the smoother it can run even for little non-cpu intensive tasks.

Well, I love my N900. So it's natural to want it a bit faster.
- less video playback frame drops
- less pauses during music playback while multi-tasking
- less frame drops in game emulators
- smoother multi-tasking in general

I wish you can increase the RAM it has to 512MB. That would actually make it a lot smoother during multi-tasking. But we know that's really impossible. So we are clutching on straws here to at least bump the cpu a bit faster.