After reading through this thread, I decided to try the 125-900mhz kernel, as I don't recall seeing anybody with problems with 900mhz (yet).

I've had no stability issues at all, most of the transitions and applications are snappier (not they were ever sluggish, but still...). I used to wait for 5 seconds before contacts showed up when I clicked contacts from within the phone application, now it's down to 3 seconds! Linear speed improvement!

The only problem I had so far was that flash support in Firefox became very slow with the 900mhz kernel, and the problem was solved when I uninstalled and re-installed Firefox. Now Flash works almost as good as in MicroB, and I'd say it works PERFECTLY with MicroB.

As the idle speed is now 125mhz as opposed to 250mhz, I believe there would be next to none extra strain to the battery. We'll all see if there will be any negative effects of overclocking the n900 in the mid to long-term but so far it seems the device handles the extra speed just fine...

My thanks go to everybody who contributed to this thread!