Xbox 360 developer kit spec http://www.pcreview.co.uk/forums/xbox-2-sdk-released-g5-r350-based-t1994129.html
"The big news to us is that the XBOX 2 SDK has been seeded to developers on
dual Apple Power Mac G5 systems running a custom Windows NT Kernel.
The Apple Power Mac G5 is based upon two of IBM's 64-bit Power PC processors
and features ATi RADEON 9800 Pro (R350) graphics. However the R350 is
believed to be an interim solution and will, in due course, be superseded by
the forthcoming ATI R420."