The X1800XT is supposed to be available on November 5th, with Crossfire X1800 cards available sometime in November. At least that's what ATI has been saying. Benchmarks show the X1800XT as very competitive with the 7800GTX. Competition and a few months' time will undoubtedly lower the price of G70-based parts, and today's reports are that NVIDIA will release a 512MB 7800GTX in early November (which could also exert some downward pressure, but probably not too much).
Beyond that, the rumors are that ATI's next PC GPU will be based on the XBox360 GPU (no, really). This would probably be in the 12-24 month range, but nobody outside of ATI's engineering department really knows (assuming *they* even do). The architecture seems to be moving towards fewer pipes but with far more pixel/vertex shaders, which could give some interesting performance numbers. Overall fillrate might not be as high as a card like a 7800GTX, but in shader-heavy games, it could be very impressive.