Today, AMD rolled out 6000M series along with Brazos APUs. TDPs are unknown which makes speculation harder but following Apple's GPU offerings in the past, we'll probably see 6950M in high-end and 6750M or 6770M in mid-level iMac.
6950M comes with 960 shaders compared to 800 shaders found in 5850M. Cores will run at 580MHz which is 45MHz slower than in 5850M, and memory also runs a tad slower, 900MHz compared to 1000MHz in 5850M. However, the biggest upgrade is 256-bit memory bus width which will help when dealing with high resolutions such as 2560x1440 used by 27" iMac. In terms of raw performance, 6950M seems to offer 1.1TFLOPs while 5850M offers 1.0TFLOPs, so not that huge upgrade.
As for 67xxM, it features 480 cores compared to 400 found in 5730M. Clock speeds haven't been specified more than that they are 500-725MHz. I would expect 6750M to have clock speed of ~650MHz and 6770M will likely have 725MHz. Memory speed will be 800-900MHz so a small improvement over 5730M's 800MHz. The best piece of news is that 67xxM comes with either DDR3 or GDDR5 so Apple will likely have to use GDDR5 for iMacs. 5730M provides 520GFLOPs and it's reported that 6600M and 6700M will provide 480-696GFLOPs so it will likely be bigger upgrade than 6950M, though it depends on what GPU Apple will choose.
http://www.amd.com/US/PRODUCTS/NOTEBOOK/GRAPHICS/AMD-RADEON-6000M/Pages/amd-radeon-6000m-series.aspx
6950M comes with 960 shaders compared to 800 shaders found in 5850M. Cores will run at 580MHz which is 45MHz slower than in 5850M, and memory also runs a tad slower, 900MHz compared to 1000MHz in 5850M. However, the biggest upgrade is 256-bit memory bus width which will help when dealing with high resolutions such as 2560x1440 used by 27" iMac. In terms of raw performance, 6950M seems to offer 1.1TFLOPs while 5850M offers 1.0TFLOPs, so not that huge upgrade.
As for 67xxM, it features 480 cores compared to 400 found in 5730M. Clock speeds haven't been specified more than that they are 500-725MHz. I would expect 6750M to have clock speed of ~650MHz and 6770M will likely have 725MHz. Memory speed will be 800-900MHz so a small improvement over 5730M's 800MHz. The best piece of news is that 67xxM comes with either DDR3 or GDDR5 so Apple will likely have to use GDDR5 for iMacs. 5730M provides 520GFLOPs and it's reported that 6600M and 6700M will provide 480-696GFLOPs so it will likely be bigger upgrade than 6950M, though it depends on what GPU Apple will choose.
http://www.amd.com/US/PRODUCTS/NOTEBOOK/GRAPHICS/AMD-RADEON-6000M/Pages/amd-radeon-6000m-series.aspx