Everyone's favourite source *cough* TheInq *cough* has posted a few details here. A few quotes:
LW.
It will be officially announced at CeBIT in Hannover, Germany on March 12, 2003.
Specifications of the new chip are being kept under extremely tight scrutiny because of the tight competition between the two rivals. It would give too much lead-time to overclock one chip just to paper launch a product and claim the performance lead.
Many facts are already known. The R300 will be built on a 150 nanometer process. It is basically an overclocked R300 with eight rendering pipelines and two texture units per pipeline. Graphics cards powered by the R350 VPU will be equipped with DDR-II memory with 256-bit bus.
LW.