Aha now you getting into another problem and that has now i believe been thoroughly been proved conclusive that mobile's do indeed cause bone damage to the middle ears.
But anyhow just something else to add, remember the N900is designed to run at 600mhz so therefore the capacitors are worked out by CAD to be of a correct value for the cpu set frquency and there is yet another heat problem because the current being drawn by the components does increase and that alone heats up the caps even more than usual under an overclock situation.
If the cpu holds out then you still got cap probs to get over lol.
But we will not go into dry joints shall we lol.
All in all its a must to create some ventilation if you want your device to last under this situation.
Your biggest problem is current drain = heat.