The CPU in the Xbox was a custom Pentium 3 running at 733MHz. The 'custom' part of this was that that the Pentium 3 in the Xbox was that it had only 128KB L2 cache instead of the usual 256KB. This allowed Microsoft to buy them at a bit of a discount and Intel to shift a few more CPUs.
- Intel® 64 and IA-32 Architectures Software Developer Manuals (Sorry, I can't find just the documentation for the Pentium 3. Enjoy 4600+ pages, half of which is irrelevant!)