According to reports, the next generation Xbox chip will be ready by the Q1 of next year. The card will be similar in architecture to R520, the upcoming desktop chip, which is code-named 'Fudo'. One major difference is the integrated frame buffer. It is believed that the card has at least 16 pipelines and 2 texture memory units, which should give almost twice as much power as the current X800 cardsthat have only one texture memory unit. The main benifit will be for games that utilize multi-texutring, which pretty much includes 99% of most games today. Performance might be more than 6 times faster than R800.
More information can be found here.