Nvidia has decided to depart from its current product naming strategy and introduce a completely new sequence number for its upcoming GT200 chips.

The new GPUs, also knows as NV60 graphics processors, will be called the GTX 260 / 280, according to an The GTX add-on now moves to the front of the sequence number and we assume that future mainstream parts will receive a GTS or GT name, while low-end entry-level parts will end-up with the SE name.
Source: TomsHardware