I don't know technically what the difference is between the 8950 and 7950 but mine runs rock solid at 1100/1575, and peaks at 150 H/s.
However I got a few Gigabyte 7950 and those can only run at 1050/1400 since anything higher and its unstable, no idea why.
RAM cooling can make a difference for overclocking. For any new card purchases I look for the RAM to be cooled by the main heatsink, or at least a clear airflow path from the fans over the RAM.