OK,First of all, just which 3 Celeron are you running. From the speed you're already obtaining, it sounds like you have the 533A. If that is the case you really shouldn't have any difficulty using the 100 MHz FSB, but I am not familar with your board. The name sounds more like something for AMD CPUs. As far as overheating, you might obtain a motherboard monitor for that. Try asearch and download one. There are several. Also, You might consideran inexpensive cooler like the Thermaltake Golden Orb. I have used them, and they work quite well considering that you would be out of pocket only 15 dollars at the most. That includes shipping. It is really quite common for the 533A to do 850 MHz without many problems--you might not have to worry adjusting the voltage either. 1.5 or 1.6 volts should be plenty. Good luck!
3dRaul