I have seen many people overclocking their 5850s to 800+/1400+ on gpu and memory.
But I cant go beyond the 775/1125Mhz. this is the maximum I get through the ATI overdrive ( and even with MSI afterburner).
I will attatch a screenshot.
I am using MSI twin frozr 5850
Also, keep in mind that OCing is very system specific in what you can do on your system vice how well someone else did with theirs. There are bazillions of factors to be considered for each individual case.
Key things to consider are airflow/heat/power? What make/model of PSU are you using? What else are you OCing? How are your temps? Etc.....