it depends whether the next xbox will use a 'powerpc' architecture or not.
Xbox (Intel CPU) -> Xbox 360 (IBM PowerPC) = can't play original Xbox games, unless you do some 'emulation' for them.
Xbox 360 (IBM PowerPC) -> Xbox 720 (IBM PowerPC) = plays Xbox 360 games flawlessly.
but i prefer that MS use an Intel CPU for the next Xbox, IBM sucks.