The original R400 was apaprently held up by manufacturing tech (PCI Xpress and the actual chip config, combining pixel and vertex shaders, like on the NV40), so it's name was changed to R500, and it will be released later than planned. In its place is a new R400 (also known as R390, R420, and Loci--before the company decided to settle on R400 again, though it's NOT the same as the original R400) that looks to be a suped-up R300/350. Current clues are that it'll be 200M transistors, double that of R300/350. That alone can account for a doubling of performance--if not in plain rendering, then in AF+AA rendering, and possibly in shader-intense rendering.