The HD 6750, codenamed Barts Pro, features 280 VLIW-4 shader clusters (1120SP), a TMU count of 56, a 725MHz clock speed, and a 1GHz memory speed. The HD 6770, codenamed Barts XT, features 320 clusters (meaning 1280SP, though previous leaks have suggested 960SP and 1120SP, so we're not holding our breath). It also has a ROP count of 32, a 256-bit memory bus, and a TMU count of 64. The core clock is a whopping 900MHz, making it the fastest stock speed of any discrete GPU, while the memory clock and thus bandwidth is at 134.4GB/s.
As you can see, the chart compares the specifications of the upcoming Radeon HD 6700 series to the previous Radeon HD 5700/5800 series. Expect more leaks as we get closer to product launch!