im not sure about the gt's core/mem clock but i believe they are talking about shader clocks which are separate from the core and memory clocks.
the gts has a core speed 500 mhz and a memory speed of 800 mhz (1600 mhz effective) while the r600 has a core clock of 740 mhz and a memory clock of 800 mhz (1600 effective).
however i don't know what the shader clocks are for the r600.