9800GTX is the fastest single-GPU solution at the moment true, but that's only because the 8800 Ultra has been discontinued. The Ultra beats it in enough things for it to still hold onto the crown, imo.
The GT200, which is speculated to be the 9900 line, is rumoured for Q3 of this year. I wouldn't expect it before September, and maybe delayed until later, but that's just my personal speculation. The 9900GTX is hoped to be single-GPU and as fast as the 9800GX2, and the 9900GX2 is expected to be the new top performer.
R700 could come at any time. Most expect before the GT200s, but who honestly knows. Ati says 2008, that's all we really know. Speculation is multiple cores on the mid and high end models. Performance is unknown, but rumours are saying closer to a 50% increase rather than a 100% increase which some generations have had.