I have a 6800GS,
standard clocks are 350mhz for core and 1000mhz for memory.
Using Coolbits, I can only increase the core to about 425, and the memory to 1070, otherwise I get artifacts
The best way to make sure is to get a better cooler, though for video cards, these can be hard to fit.
There is a limit though to how much a component can overclock. Its not just heat that matters. There are physical limitations.