vanshardware.com shows us a dissected XBox.
Here are some interresting bits:
"For many months, the official data on the Xbox website only stated that its CPU would be a 733 MHz Intel part. As we can see in the photo below, it appears that the CPU is simply a 733 MHz BGA Mobile Celeron mounted directly to the motherboard."
"There doesn't appear to be any way to easily overclock the Xbox. Those few souls bound and determined to do so can find one certain route by removing the clock crystal and supplying an external clock signal. Be aware that every aspect of the machine will be sped up as well, meaning that game play will also be faster -- perhaps too fast if the adjustments are not made carefully. Also, since the entire system would be overclocked, headroom is not likely to be great."
News source: Van's Hardware