the original xbox was basically a x86 pc in a htpc case.

the xbox360 has a 3-core powerpc cpu, and a G5 would probably be the most straight forward platform to develop on (rather then emulators or cross-compilers).

btw, both the wii and the playstation 3 use powerpc related cpus (the ps3 cell being the really odd one, using a powerpc core and up to 8 special cores).