well its not really hard at all to understand if the XBOX is indeed running a celeron chip or full Pentium. When your talking about console games its a whole different ballgame than with regular pc games. Console games are written and fine tuned to run on that specific hardware and squeeze every ounce of juice out of it (at least they try, as time goes on programers learn the hardware better to squeeze out more juice, perfect example is first gen playstation2 games vs current playstation2 games). PC games cant do that because there are way too many hardware configs and you cant code to support everything or max out everything cause everyone doesnt have a P4.
The processor in the XBOX dosent handle graphics tasks much anyway, the GPU does and its a geforce3.5 level card I say 3.5 because it has more features than a retail gf3 but not equal to a gf4 card. The processor handles AI and non graphics chores. In practice you can use it for whatever you want it to do in your engine, but that would be dumb and your game would probably run like crap.
Halo runs like crap on the PC because it was ported to the PC in X months and rushed out the door with little or no time for optimizeing. This is a fact. The other main reason is because they had to support i believe at least geforce2 level card. This may have changed but this is what it was originaly. So a lot of things may have had to be done using software where before it was using hardware. The xbox does not run games at 320x200 it runs them at fullframe NTSC spec and it also has support to run games at HD resolutions, and its not faking it. You can also run xbox game at regular pc resolutions.
weather or not the xbox has a celeron chip or full pentium, im not sure. I wouldnt doubt either. But i think it may be a celeron chip, cant really remember right now. But i can have a concrete answer about this when i get in to work tomorrow.