I flashed the 900MHz with stock DSP kernel to my N900 about 36 hours ago, and had no problems whatsoever in terms of stability.

A few hours ago, I tuned the CPU down a little to 800MHz, just to be on the safe side as it tends to get very hot in summer around here.

You can never tell for sure, but I doubt I'll ever have any problems with the 125-800MHz settings.

On a side note, 900MHz is noticeably snappier than 800MHz, but I think I can live with the difference )